One can still measure the actual character width using
> window-text-pixel-size and take it into account.

 That's correct. width is pixel width returned by (posn-at-point) and
w is (font-get
(font-at (point)) :size).

This assumes that, e.g., #x2001 takes the same horizontal space as the
> SPC character (or the same as a column)?

The code is only assuming what the specs prescribe. It makes no guarantee
for non-standard fonts.

The complete code in action, if anybody is interested, is here
https://gitlab.com/atamariya/emacs/-/blob/dev/lisp/cedet/company.el#L2562 .