LSD is one of the most faked drugs by far. There are communities whose sole purpose is to find the real amongst the fake. It’s so bad that you’re more likely to get fake LSD than real… Cocaine is always somewhat contaminated but it’s comparatively very easy to get what people consider a quality product.

Is most LSD sold on dark web markets fake?

It is not, he's listening to the hippies that want the "real LSD". "real LSD" means weaker tabs, they weren't as strong back in the days, 200+ug gets you too high.

Oh there's plenty of dubious shit being sold as LSD. One of the most common has been 25I-NBOMe, which has the unfortunate ability to be lethal at a dose of a couple blotters(a fairly common dose for LSD).

If you're purchasing through a reputable marketplace from a seller with good reviews this won't happen, if you go on the streets maybe.