Yes, it is quite true - sometimes a foundry can't even duplicate the previous batch of alloy exactly.
It is also true, however, that when tire shops are throwing old wheelweights in buckets, they don't sort them into various buckets according to manufacturer, style, weight, alloy etc. and as a result things pretty much average out by the time the bucket gets picked up, no matter what shop you're in. Having been melting down wheelweights in batches of a couple of hundred pounds at a time the last thirty years or so, I have made the startling discovery that the hardness of the resulting alloy is pretty much always the same, give or take 1 BN point. At least, that's what Veral Smith's hardness tester tells me.
It is also true that scrap wheelweights are hard enough as is for mild use in centerfire rifles and pistols.
Finally, and perhaps most importantly, it is also true that while the proportions of Pb, Sn, and As in wheelweights can vary from manufacturer to manufacturer, those differences do not affect the ability of wheelweight alloy to be heat treated to specific hardnesses using one standard heat/time chart.
One could of course pay for an assay each time a load of scrap WW gets turned into bullet alloy - but the resulting information would be pretty much pointless in that it wouldn't affect useage in the slightest or how the bullet caster heat treated their bullets.