There's some misinformation here.
First, there's no such thing as a UEFI-only graphics card. If it's UEFI compatible, it's because it has a UEFI GOP image added to the VBIOS. Which is essentially tacked on to the end of the Legacy image. Making it what's known as a Hybrid VBIOS(as in both Legacy and UEFI, 2-in-1). Which means you can use a motherboard's UEFI GOP features(fast boot, secure boot, etc.) with the card installed as the primary GPU.
Second, regarding that reddit post, that's just as much about GPT vs. MBR, as it is UEFI vs. Legacy. What I'm getting at there is, as far as using UEFI GOP features, you need to have your OS on a GPT partitioned disk. Since UEFI only supports GPT. But you don't need to use any UEFI GOP features(as in have them enabled) with a GPT partitioned disk. And you don't need a graphics card the is UEFI compatible, unless you want to use any UEFI GOP features. Meaning, you can run a Legacy-only card on a UEFI system. You just need to disable UEFI GOP features. And not even all of them really. Just secure boot. Other UEFI GOP features will not work correctly(like causing a black screen during boot, instead of seeing the Windows boot screen, with fast boot enabled). But the only thing keeping a computer from booting with a Legacy-only card will be secure boot. And UEFI or Legacy mode has nothing to do with Windows recognizing/detecting the card once booted. That can only be caused by something else entirely(what that is, is the question).
Bottom line: This has nothing to do with a UEFI-only graphics card. Since there is no such thing. And there's no real need to use Legacy mode to boot with a Legacy-only graphics card. You just can't use UEFI mode, with secure boot enabled, with a Legacy-only graphics card. And there's no reason whatsoever to use Legacy mode to boot with/use a UEFI compatible graphics card(unless you need to use Legacy mode because the OS is on an MBR partitioned disk).