I also only use strictly throwaway emails. If a website finds it fishy, they will tell you while you are attempting to create an account (in which case I just dont use a random string and use random words instead). Back when I was in the coversion process, several accounts were frozen when i switched from a real to throwaway email. Each time I just emailed customer service with the custom email, they realized I'm not a bot and as long as I could pass dual verification (which I can because I'm me), they unfroze my account and occasionally apologize.

How would Discord know it was a throwaway email?

I use Fastmail + their masked email feature with a legit DNS, not a random one time email gen site.