You can put a 125 w cpu technically in 7090 sff and motherboard will boot with it.  The sff and mt psu are interchangeable with same physical form factor and connectors for motherboard.  The practical issue of 125w cpu in sff is that sff has a low profile cooler which is inadequate to cool a high power cpu, while the MT having a bigger case can use Dell premium tower cooler for better cooling.  So it is not the sff motherboard or psu but the cooler that is not rated to cool a 125 w psu.  If you try it the cpu fan will be very loud most of the time trying to cool and cpu temp will be unmanageable causing throttling.
Hello!   I bought an Optiplex 7090 SFF. (It has not arrived yet). I see in the specifications page that the SFF can be ordered with 65W CPUs only but the MT can also be ordered with two 125W CPUs (i7-11700K and i9-11900K). Is this just a marketing decision? Can I put a 125W CPU into the SFF too or is the motherboard/PSU combo too weak for that?
Optiplex 7090 SFF vs MT 65W vs 125W CPU (11700k and 11900K) question