It's useful advice, for those who don't want to get burnt out on acid. So many suffer the 'grim meathook reality' as Hunter Thompson wrote. I mean psychedelics do induce spiritual experiences when the set & setting is right, but too many over-indulge and think they're the Messiah / Second Coming after it all, which is dangerous.
Also: the west has no shamanic centers where we can do psychedelics safely, apart from the Native American churches where you can do peyote rituals without getting arrested. Sadly the men in white coats are the closest thing to a shaman you can get in the West, and we sometimes need a sitter/guide especially with high doses, preferably someone who has done it before multiple times, and can calm you down incase you start having very pronounced anxiety.