It is set up for Aurdino coding, including their bootloader.

Which is fine. And has opened up some sort of coding to a great many.

But if you want to use C, rather than their interpreter, and develop compact code to run on a minimalist embedded device, best off starting in Atmel Studio and learning the hard way I think.

For which I think you will need an USBasp, not an Isp, (or the basic USB connection baked into these things, with their bootloader, that takes up a lot of space).

I have no idea how tricky it might be to return these things to vanilla Attiny 85s, but it might even require a high voltage programmer to re-set some fuses.

They do look good though, I suspect the one you linked to, and many others, are now knock-offs. The real ones from the original designers cost much more, and are now unavailable. But still less than a cup of over-priced coffee, or half a pint of “craft” beer. Sad.

Take a look at e.g. Julian Iletts channel for some background. This is a very old video (2014) but explains a few things.

And yes these MCUs have been around for a long while. Consider them mature (extremely so).

And take a look at https://www.youtube.com/watch?v=0B88dsW93SA

I only dabble in firmware enough to exercise my hardware designs with test harnesses, the clever work is done by proper skilled developers.