They’re referring (I believe) to the screenshot right at the top of the article, which includes this absurd calculation:

border-radius: max (0px, min(8px, calc( (100vw - 4px - 100%) * 9999)) );

My guess (hope!) is that this is not ‘serious’ code, but padding for the sake of a screenshot to demonstrate that it’s possible to use each of these different features (not that you should!).

Onlyoffice does now have an ARM version, I run it on a raspberry pi 4 (integrated with seafile), and it works fairly well. Can’t vouch for how much power it needs, to say if a pi 3b+ will be enough, though. Pretty sure it’s lighter than collabora, since more is done on the client side.

I agree with the other commenter that suggested cryptpad, though. If all you’re after is a Google docs like collaborative experience, cryptpad is brilliant, and much more resource friendly. (The office editor it uses is also a slightly modified Onlyoffice, so almost exactly the same feature set)