Below is the published spec's from the Dell manual.  I tried to get the Crucial list for this model but the Crucial site is not working properly.  Keep in mind its best not to mix memory, even if its the same speed.  Best to only install same memory from same vendor.  

Would https://www.amazon.com/dp/B00Q0MJJWW/ref=cm_sw_r_cp_apa_i_wAUJCb13MFQV8 be compatible with the Dell optiplex 990 mt?

The crucial site is now up, below is their listing.   Note they list 1.35V memory and the Amazon memory you link is 1.5V and may not work or work properly.

https://www.crucial.com/usa/en/compatible-upgrade-for/Dell/optiplex-990-mini-tower

The ram is INTEL compatible,  LOW Density,  Ram.

https://www.crucial.com/usa/en/optiplex-990-desktop/CT2348234

DDR3 PC3-12800 • CL=11 • UNBUFFERED • NON-ECC • DDR3-1600 • 1.35V • 512MEG X 64 •

I recently updated the bios to ver. 24 and installed 32gb ram with my dell Optiplex 990 SSF. Everything is running fast and good now. Photoshop and lightroom opens supper fast.

Here is the link to the Ebay buy: https://www.ebay.com/itm/Transcend-8GB-2Rx8-DDR3-1600mhz-PC3-12800-UDIMM-Memory-FOR-PC-Desktop/223839329356?ssPageName=STRK%3AMEBIDX%3AIT&_trksid=p2057872.m2749.l2649

-Scot