Honestly the too much cynicism is like a plate full of coleslaw reference sounds a lot like those tactics cults use to shut down critical thought. Similar to toxic positivity or the like. Basically he's saying 'don't think about it too much'. I don't know the full context of the reference since Ollie is catastrophically bad at engaging with citations in his videos so I could be misunderstanding it.
So this quote is something he made up. David Hume never said anything about coleslaw because coleslaw had basically only just been invented in the 18th century.
David Hume's perspective was kinda that rationality has limits - where other Enlightenment writers were trying to view the world through order and reason, he was more like "It's impossible to know anything empirically because everything is filtered through our senses and so we can only make assumptions based on what we're experiencing, and we fall into patterns based off previous experiences". So for example you can't empirically prove that sticking your hand into a candle's flame will hurt you, you just assume based on prior experiences with candle flames that usually that hurts you. But also that a literal child will work that out, so not being able to empirically prove something isn't necessarily a big deal?
Relatedly there's the concept that morality isn't really about rationality, but more about good and bad feelings. Cheating isn't wrong because of a rational thing, but because it causes hurt, and your "operations of sympathy" (basically empathy) make that feel bad. We can use rationalisations to describe why things are good or bad, but they're just post hoc justifications and it's really just about feelings, which aren't always rational. This is kinda like utilitarianism - he thinks celibacy is bad and a denial of our human feelings, but also being super horny all the time is bad because it makes society less stable.
I'm not explaining this well because I'm not a philosopher... but basically "Reason is, and ought only to be the slave of the passions, and can never pretend to any other office than to serve and body them" - in other words, the opposite of "facts don't care about your feelings".
You could use Hume's approach to argue that there's no meaningful definition of what a man or what a woman is, but that's not really what he was arguing. He argued that there isn't really a consistent "self" - when you try and percieve yourself, you're perceiving how you are in the moment; maybe you're hungry, maybe you're upset at something your boss said, maybe you're in a good mood because it's sunny outside. You're a bundle of perceptions and the pattern of those perceptions creates an idea of a personal identity, but there's not really an empirical "you" - so you could also take that approach to argue trans women are not women in any meaningful way and they're just perceiving themselves as women.
He was mostly arguing that if you're a stickler for empiricism you're going to tie yourself up in circles, and we have to accept that humans aren't rational and are full of contradictions. He certainly wasn't of the opinion "if someone says they're a lesbian then just accept that and don't question it". He was a proponent of being sceptical of dogma and orthodoxy, arguing that religious belief is usually because of fear and the positive feeling of being in a congregation rather than because God is real.
Ollie's main falling down in his argument is he thinks transphobes go "how does someone know they're a lesbian" and not "this person thinks they're a lesbian but they're not".
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