monged on Dylan Thomas

Tolkien, C S Lewis, there amongst the dreaming spires in Oxford with their over-mapped and over-textured mythopoeic inklings of an underlying landscape, a default Arcadia. Henry Treece, Nicholas Moore, intoxicated with Apocalypse and monged on Dylan Thomas, seeking immanence from every bud, from spit-dilutions left in brown glass bottles.

Source: Introduction to A Voyage to Arcturus by Alan Moore

Can you please explain what is meant by the part in bold?

"monged" is a british slang word which is similar in meaning to "stoned" or "wasted", usually on drugs which have a tranquilising effect (marijuana, valium, opiates etc).

https://en.wiktionary.org/wiki/monged

Moore is (in my judgement) implying that they have read so much Dylan Thomas that it has made them slow and sleepy and perhaps lacking in good judgement.

"monged" probably originates from the now-unacceptable term "mongoloid", an old word for people with Down Syndrome, implying that the "monged" person is like someone with "mental retardation". This puts it in a very unfavourable light as an acceptable word these days, and I suspect that Moore (usually a very politically correct person) wasn't thinking about these connotations when he wrote the piece.