Funny

that idea came from carl jung, and was largely informed by applying ideas from chinese texts on internal alchemy translated by hellmut wilhelm, during a time when it was all the rage for rich western fucks to go appropriating and syncretizing a bunch of shit none of them understood, often toward the end of servicing a proto-eugenicist-at-best narrative about atlantis or hyperborea or whatever the fuck.

so like my number one is that occultism of that era and the modern pop occultism that derives from it (9/10ths of your "witchy" friend's shelf full of pristine lewellyn titles) tends to be gravely mis- or malinformed at best. neo-hermeticism in general tends to be rife with this type of stuff, because theosophists and thelemites alike had a gigantic orientalism-boner for egypt.

number two is that chinese internal alchemy and chinese chemical alchemy (for lack of better terms, i'm atrocious with chinese) were/are two entirely separate disciplines, the former of which is still practiced and both of which are documented enough that it is clear they are not metaphors for each other. jung was essentially comparing apples to oranges in order to understand schnauzers.

number three is that the number one bit of advice about studying taoism that i get from chinese speakers is to avoid the wilhelm translation of basically anything, so like even the already unrelated ideas that jung was injecting where they don't belong are likely innaccurate to the actual tradition.

number four is that i'm currently studying medieval and reniassance magic and the notion of alchemy as a metaphor for self-therapy seems pretty foreign, and the notion of transmuting metals is not only clearly very literal but a consequence of the prevailing model of physics of the day.

number five is that "late-victorian-to-early-modern-era scientist thought [insert unrelated thing] was an ancient metaphor for the field he pioneered" is such obvious bullshit and jung's tenuous association says much more about jung and the prevailing cultural perception of ancient peoples in jung's time than it does about either alchemy or psychology imo.

tl;dr medieval alchemists were clearly proto-chemists, alchemy as a metaphor for spiritual or psychological development is a novel creation of one or more coked out victorians because it made them horny to connect their pet ideas to the wisdom of the ancients or whatever.