I've heard about the milk thing before, it's quite common in poison control.
It used to be that the number one thing people knew to do when poisoned was to induce vomiting. However, poison control only recommends that for some poisons, and many others vomiting will make things worse.
I haven't heard of drinking milk as being explicitly bad for anything, so maybe this will become the new main tip.
I believe (and someone correct me if I'm wrong, it's been a while since I've taken biochemistry) hull cleaner is usually made of phosphoric or citric acid, and it has a higher affinity to binding to the milk's (hydrogen?) molecules than the receptors it would bind to in your body to poison you. So the acid'll bind to the milk and create some sort of solid (milk curdle) that will take longer for your body to digest, giving you time to call poison control.
Similar principle to drinking vodka if you've ingested methanol, but for that mechanism I believe vodka binds to the receptors in your body faster than methanol, and that's what slows down the poisoning.
Neither of those acids should have systemic toxicity. You'd need a lot of phosphate to get sick(around 60 grams is fatal and 4 grams is the recommended upper limit), and it is not absorbed very well in the first place (and just leads to diarrhea). Citrate, similar to phosphate, can cause hypocalcemia but you really would have to ingest a lot of it. Both much prefer being calcium salts instead of sodium salts. Usually it's for neutralization more than anything.
On the point of methanol: the treatment with ethanol is both to give much more ethanol than you have serum methanol and to rely on ethanol being a better binder. Methanol itself isn't what's toxic, it's the fact that alcohol dehydrogenase metabolizes it into toxic formaldehyde much faster than your body is able to clear it.