It's a
general statement from the main pages on the site. Screens are said to block 30-60%, depending on the type of screen, the source and UVB strength (It also depends on what many manufacturers might say or claim) and I'll rather group the smaller grid screens as obstructions. It's also a way to
suggest to people that they shouldn't keep their tanks near windows, which is something that needs to be reminded of often.
I'd much rather be extra cautious and you'd find references for larger screens on many sites that pre-date this one. I would like more accurate readings of UVB but have only come across one or two sites that collect this type of data. However, how they conduct their testing may not necessarily reflect the conditions that basic keepers will be using artificial UVB. Well, anyway, that's my reasoning for a statement like that.