Why would engineers need to use the same OS platform as the designers?

Most designers use Figma these days, which has clients for Windows and macOS, and can be used fully in a web browser as well.

Figma's valuation is a fragment of that of Adobe. Saying most designers use Figma is as distorted as saying most designers use macOS, even if both things may be true in a specific niche (e.g. tech companies in the US).

In my experience of the rise of macOS among developers, the biggest driving force was mobile app development and mobile web development because Apple refused to provide first party developer tools or emulators usable on other operating systems and made it intentionally difficult to run macOS on non-Apple hardware or in a VM. Previously macOS was largely associated with designers, not developers. This move also started blurring the lines more with designers making the move to development and (to a smaller degree) vice versa.

No, developers flocked to macOS because it runs unix and has a complete and nice looking UI with sensible design patterns designed by an actual designer instead of an engineer.

Development across the board is better on a mac for some / many people.

I think that's a fair take, it really does depend on which platform(s) you're developing for too.

And you're probably correct that Adobe has a larger share overall, but I would highly doubt that Illustrator or XD are being used more than Figma for UI design these days. Even less so, Photoshop, since it's a raster tool. Adobe was about ready to phase out XD when they were getting ready to purchase Figma.

Figma is free starting off, and their basic plans are extremely affordable compared to anything Adobe offers.