So this needs the buffer text be displayed in some window, before it
could be printed?

Yes. Plain text file would not have any font/ display information otherwise.

> It uses following API to determine BIDI classification and processes accordingly:
> (get-char-code-property (char-after from) 'bidi-class)

Hmm... not sure this is enough, except in the simplest cases. The
bidi-class of a character is just its basic attribute; application of
the full Unicode Bidirectional Algorithm can and does change the
effective attributes of some characters, before they are displayed.

Do you have some sample text for this use case? I can run a quick test.

Is this test performed on each character or only at the beginning of a
visual line?

This happens at every point where the font changes.

Apologies if I didn't understand what the code does correctly; I only
had a cursory glance on it, and there are not so many comments
explaining what it does.

No worries. I'm happy to answer any queries.