TLDR Your PSU is fine for any single GPU on the market today with a consumer (not HEDT) platform.
You're only looking at the PSU at a very high level (number of connectors, how many watts, etc.)
The S12/M12 uses a double forward high end that has what we call "hard switching". Newer graphics cards like the 2080Ti produce high transient loads during gaming, benchmarks, etc. that, at the very least, will make the main transformer vibrate with an audible noise or shut down during these transient spikes.
Add to this that the 620W is group regulated on the output side so it doesn't support the newer (S6 and higher) sleep states and the supervisor IC used is lacking some of the necessary protections.
It's just a poor choice for anything other than an entry level system.
YOUR power supply is a Seasonic X850 which is a COMPLETELY DIFFERENT platform that uses an LLC resonant front end and has DC to DC for the +3.3V and +5V. It's like comparing a Ford Fiesta to a Ford Mustang.