Hardware makers also care about what gamers use to game. 90%+ gamers on PC use Windows. That's it.

To give some numbers, Steam has 96% Windows users.

Source: http://store.steampowered.com/hwsurvey

Hardware makers unlike publishers compete more directly. So everything counts.

Like publishers, hardware makers understand Opportunity Cost, and know where they should concentrate their efforts for maximum payback.

That's why AMD actually have a team of developers who are working on FOSS drivers. And Intel as well for the reference.

No, because they still make money off the Linux enterprise/HPC/etc.

If you noticed, the main article speaks about how those "teams of developers" DON'T plan to support XWayland.

They don't need Mesa for that, their "worsktation" customers are using their closed driver. But if you missed it, unlike Nvidia, AMD have a team which contributes to Mesa directly.