and this would still not have enough thickness to it for a significant UV blocking effect?
Correct. UVc and UVb energy maybe, but ordinary window Glass filters most of that out, too, so not a big contributor to indoor fading. UVA (340-390 nanometer wavelength) will slip through for the most part. If it didn't, a paper like Epson Exhibition Fiber would totally turn from cool-white to warm-white appearance with the addition of the spray. It changes a little but not much.
Sometimes I use Winsor & Newtons classic spray and other cheaper products without any UV blockers and get all the aforementioned positive changes in contrast, vibrance, blacks as well as scratch resistance. Is there a reason to use those specialized products with premium price? Till now I can't see any difference what so ever.
I have no experience with the products you mention, but my sense is that any more expensive "varnish" that claims to have UV blockers but goes on as thin as Print shield isn't really going to be very effective as a UV shield. Besides that, fading occurs with visible, especially blue, wavelength radiation. Although not quite as potent photon for photon as UVA radiation, there's more of it in typical light sources (including daylight) and thus enough to fade the print even if all UV energy is excluded.
More often I use a UV blocking varnish/coating, MSA by Golden. Not as a spray, but in liquid form applied with a roller. With minimal practice it is possible to do thick and perfectly even coatings. In my opinion, they have the advantage of "encapsulating" the ink and maybe, of providing a thick enough layer of UV blocking by the same token.
Yes, a thicker coating increases protection (sometimes at the expense of print aethetics and sometimes not) but it's easy enough to verify the effectiveness of said coating as a UVa energy blocker. Just coat a portion of a high-OBA content paper like EEF and look at the uncoated versus coated portions with a Blacklight. If the coating is effectively blocking the UVA energy, both portions will look similar (ie.,no "glow" of fluorescence) under Blacklight.