It's not that it's less stimulating, it's that pseudoephedrine basically is methamphetamine (the chemistry to reduce it to meth is truck-stop straightforward). But oral phenylephrine doesn't work at all.

The case against DXM is nowhere nearly as good as the case against phenylephrine; phenylephrine is a scam, and DXM is a drug everyone thought was the gold standard cough suppressant, but then serious studies knocked down its effectiveness.