The sentence in the Wiki article is:
This mongrel spelling of the word 'psychedelic' was loathed by
American ethnobotanist Richard Evans Schultes, but championed by
Timothy Leary, who thought it sounded better.
The reference Wiki cites for this statement explains that
Schultes was objecting to the spelling, not the Greek derivation:
Schultes cautioned Leary that the word, meaning "mind manifester," was
appropriate, but the spelling was incorrect. The proper Greek was
"psychodelic," and Schultes was concerned lest a Harvard man be
associated with the bastardization of a classical language. Leary
suggested that "psychedelic" sounded better. Wade Davis; One
River:One River: Explorations and Discoveries in the Amazon Rain Forest p.120 (1996)
The OED gives the etymology as:
Irregularly < ancient Greek ψυχή psyche n. + δηλοῦν to make manifest,
reveal ( < δῆλος manifest, visible: see delomorphic adj.) + -ic
M-W and Lexico also label the formation as irregular.
Note that the other (non-adopted) proposal
"phanerothyme," from the Greek terms for "manifest" (φανερός) and
begins instead with a combining form, "phanero-", as "psychodelic" would.
If the Wiki contributor had written, say, unorthodox spelling instead of mongrel spelling, perhaps readers wouldn't be confused.