The reason is that a new feature level called 11_3 was introduced to the last version of DirectX 11, and a lot of games that say they are DirectX 12 only are in fact only using the 11_3 feature level. 11_3 supports limited features from the 12_0 and 12_1 feature levels introduced with DirectX 12. So while it is technically possible to get a DX11 card running DX12 games, not all of them will be rendering properly if they lean heavily on newer features that require the hardware to support the new assembly translation. Notable features that the 11_3 feature level lacks compared to DirectX 12:
He has it running Hitman III and in my opinion pretty well for a 11 year old graphics card with poor driver support. He has it running at 1080p on Medium settings. This is a directx 11 card and it's not suppose to support directx 12 games.
At the very beginning of the video you can see the game is running in DirectX 11 mode, not 12.
Am I the only one impressed by the performance of a 11 year old card with barely any driver support? It plays games like Resident Evil 2 remake at medium settings at 1080p and I'm sure it's capable of playing Resident Evil 3 remake at similar settings. Aren't RE 2 and RE 3 remake the same engine or is RE 3 engine tweaked and a little more demanding?