That wouldn't comply with the license terms, which require the first two paragraphs (The above copyright notice and this permission notice) to be included in copies. LibreJS tags don't include that.

We have SPDX licence expressions as well though.

Sure, but in fairness LibreJS (like most FSF projects) pre-dated SPDX by at least a year or two. If they redesigned it today, it would use SPDX.

If only they kept their eye on the ball and stopped trying to flag "non-trivial" JS.

Not too late to switch I think, sometimes people should just consolidate efforts.