No DLSS isn't fully supported on RTX 20 & 30 hardware, it lacks frame generation, so it's not equivalent, and multi-frame generation is only available on RTX50 series, therefore it's not as backwards compatible as you make it seem.
nvidia dot com/en-us/geforce/news/nvidia-rtx-games-engines-apps/
FSR4 INT8 comes to devices like PS5/PS5P, Xbox and Steam Deck first before other of their generation, because they are known set configurations without a myriad of XYZ variables; so they are easier to build for and troubleshoot as a development platform. Unlike the various tiers of GPU CUs, VRAM, CPU, RAM, Storage configurations for add-in card setups.
It comes to RDNA 3/3.5 devices first (PS5P before PS5/Xbox) because the INT8 throughput is vastly superior. Makes sense to start at PS5Pro and work way down when PS5Pro can run INT8 @ 300 TOPS (about 50% more than 9060 close to a 9070, and similar to 3090/4070), whereas XboxX @ 24 TOPs, PS5 base 20 TOPs are far off the mark.
Also PS5 and Xbox outsold discrete add-in cards by a wide margin, so again focus on larger install base, all with iGPUs being the runt of the litter and not powerful enough to get focus other than as scraps leftover from the others.
That is as it has always been, where the capability is first, then ported to the major market segments, then trickle down to smaller individuals. If you've gamed for more than a weekend, then you'd know this by now from the old nVidia & Ati days with DX and OGL feature set support.