There aren't really rules to inktober. Inking digitally isn't "against the rules."
This one tweet is just the guy who started it saying "Here's my reasoning for why I think that inking digitally defeats the purpose of why I started inktober."
He even says that no one's going to stop you from doing it any way you want to! I did some pieces digitally for Inktober last year. I'm against acting like traditional media is inherently superior to digital media, and he absolutely implies that here and I don't agree with that -- but I don't think it super matters if the guy who started a drawing challenge has an opinion about the best way to do the challenge.
I think "If you're able, I'd encourage you to try using pen and ink for this challenge, in order to practice working with a permanent medium!" would've been a better way to word a similar sentiment.