The answer is that if the system won't operate at full power with the adapter directly connected, neither will the docking station.

They wanna sell their chargers and docks, so no it won't supply more than 100w via a third-party type C charger. You will get error "AC adapter type cannot be determined" Also it will reduce processor power state whenever it's plugged in. Being a photo editor you will need all the power it can delivered to that RTX. 

@XPS_Man​ That is amazing!  I had no idea that my CPU / GPU performance was being reduced when I plugged in the non-Dell USB C PD 3.0 or 3.1 charger.  The whole idea behind the PD standard is to not have proprietary bricks stuffing our briefcases and bags.  One for all.  Everything is going to USB C PD 3.1 charging for perfect handshakes between device and charger up to 240 watts.  I am shocked that Dell would fight this or deliberately limit it.  Anyway, if that is true (that the CPU / GPU performance is reduced) if the PD charger is plugged in, that means I will unplug it while in Photoshop or Lightroom and go off battery power, then stop and charge up the battery with the allowed 90 watts.  
Are you sure that Dell is not honoring the PD 3.1 standard on the three DP 4 ports on the new XPS 16?  That would not be good.  I see no advantage in that for them.  It is clawing their way backwards.  PD 3.1 will provide a perfect handshake between the Dell device and any PD 3.1 USB C 140 watt or above charger.  Those are becoming plentiful and cheap.  

The Advantage for Dell and other OEMs is to sell their branded chargers and more importantly since these XPS machines do not have USB A ports anymore, it boosts their sale of docking stations.  Only a Dell branded dock can supply 130w of power required for XPS 15 and XPS 17. So a cheaper TB/Type C dock is out of question. 

We see the same issue here in India for different devices Cellphones for example. My 120w cellphone charger works fine with some other brands and doesn't with Some. 

This isn't purely an issue with Dell not accepting non OEM chargers, the issue is your charger cant output the correct voltage/amperage.

All the 140W chargers I've seen use PD 3.1 which can utilize >20V, and they hit 140W by outputting 28V @ 5A. 5A is the typical limit for chargers though thats not a hard limit.

The Dell doesnt have PD 3.1, it uses PD 3.0 so has a maximum voltage of 20V. The two charging amperages it allows is either 6.5A (for 130W) or 4.5A (for 90W).

The decent USB-C charging cables also are pretty much all rated for 5A. When you see 240W, what that really means is 48V @ 5A.

Since the Dell cant accept 28V, you're knocked down to 20V. Since your charger can't output more than 5A, you're knocked down to 4.5A. Total charging is 90W then even for a 140W rated USB-C.

However, that's not to say that there aren't chargers/cables out there that can output 6.5A @ 20V, its just not typical and I don't think anyone would bother to crank up the amperages for this one edge case.

@XPS_Man​ 

I have the same charging Problem even with an original Dell Monitor. Dell U4025QW in theory it should have a 140 W Charger but does not charge my Dell XPS14 9440 with full speed :-(

The original PSU has 100W 20V*5A. It seems that the problem is also with the charging protocol?

I tested several GaN PSU´s Anker, Ugreen etc. no one is working...

I'm really disappointed with that...

Is there any Solution for that? I thougth even an original Dell Monitor with Charging should work but it isn't.....

@CBA2024​ I just got this charger.  It is rated at 6.25 amps (even thought it says 6.5 amps) at 20V, or 125 Watts.  It is the closest I could find.  It does charge the laptop but I'm not sure if the CPU is throttled.

@CBA2024​ Yes I just discovered this 140W thunderbolt 4 Dell monitor doesn't deliver 140W and I get weak charge warnings - bizarre a Dell monitor is not compatible with a Dell laptop.

I have an XPS15 and I also purchased a smaller, lighter USB C power pack for travel - to be hand luggage friendly I risked a 65W model, and it seems to keep my laptop charged without any problems (albit it more slowly). But I am not running it at full power or doing anything too demanding, especially when travelling.

The most a Thunderbolt 4 connection can deliver is 100 W -- and most are capped at  90W.

I have charged my Dell XPS 15 (130 watt) laptop many times overnight with an old small 15 watt phone early version PD charger.  It is slow, but any PD charger will charge the Dell even at the slowest possible throttle back speed. It just takes all night.  But it saved me in a jam 2 years ago.

However, my new 140 watt GaN5 PD 3.1 PPS charger still will not properly handshake with the Dell all the way to fully powering it during use.  But it does charge it pretty fast so you can go off battery power and then just charge it back up.  I hate traveling with the giant Dell proprietary non-GaN gigantic old brick.  I want my small GaN multiport chargers to charge everything I pack - phone, tablet, shaver, earbuds, laptop, camera, kindle reader, etc....  But remember, you have top make sure you have the right cable and not too long....