"This law doesn't prevent consumers from buying X, it prevents vendors from selling X"

The bulk of your comment is just an explanation of why you think sumptuary laws can be good (which may be correct! Not arguing that), not a refutation of the idea that this is a sumptuary law

It doesn't prevent the sale of X - it incentivizes vendors from producing X + X^1 + X^2 + X^3 + X^4 etc with the additional production going to pure waste.

Look at the nature of production to meet demand before and after fast fashion .. there's been a sea change to not simply meet demand or provide a little additional choice but to over produce as a means of changing consumer behaviour to milk out every cent with a by product of magnitudes more landfill than has previously existed.

What, concretely, is the X in this case?

I used 'X' per se specifically because I am making a generalized example. The proposition is absurd for all X.

The law is clearly not preventing vendors from selling X, its preventing them dumping it if they can't sell it.

Not sure how people can't see this difference. Preventing companies dumping or disposing of their (harmful) waste (in an unregulated manner) is pretty common practise and generally agreed upon good thing.

Read the article, or any of the stuff I quoted. This is more widely-reaching than "you aren't allowed to throw things away" (which would also have a sumptuary effect, although more indirect).