P2415Q, Hz, FPS questions

Hi,

I just bought an P2415Qb second hand. The plan was to connect my PS5 and Xbox Series X from time to time.

Firstly, the screen refused to give me higher resolution than 1080p, but then I found and article from Dell =
Setting Up the P2415Q / P2715Q Monitors with HDMI 2.0 that Support 4K x 2K 60Hz

I did this and voila! Now it's 4K on both my PS5 and Xbox. 

The thing is, both my PS5 and Xbox informs me that the screen is 60Hz. But reading up on Dell's article the screen shouldn't support this. Quote from Dell:
"Dell P2415Q and P2715Q displays purchased before March 2016 (Versions A00, A01, A02), only support HDMI 1.4 and provide 4k x 2k (3840 x 2160 pixels) resolution 30 Hz using the HDMI cable connection. 4k x 2k 60Hz is available via DisplayPort connection on these displays."

My monitor was manufactured August 2015. But I think it was sold in the stores sometime in 2016. 

I've tested some of my games and they seem to run in 60FPS. When I change between "favor resolution" (locking the game to 30FPS) and "favor performance" (unlocking the FPS, so it can run in 60FPS), I can notice some difference. But I'm used to gaming on my 55" LG TV so I'm having a hard time to notice if it's 60FPS or just my mind playing tricks on me.

I've tried to search up my monitor with Service ID or serial number, but I get 0 results on Dell's support site.

But if my PS5 and Xbox Series X shows me that the TV (monitor) supports 60Hz, maybe the screen is "after March 2016"-model? Even though it was manufactured August 2015?