Surely it's possible to block the paths to the ad on the site itself.
Not with DNS blocking, which is what the pihole is
Pi Hole just blocks on DNS level, I believe.
The problem is that it will refuse to play content if it can't play the ad.
Pihole is only intercepting DNS resolve requests. Those only resolve the domain itself. If they don't use a different domain to serve ads, then it can't be blocked that way.
Good thing the whole site is a bug, though I rarely end up on YT by accident since videos are marked in web etiquette.
For twitter.com a filter option here on HN would be nice but a domain blocker might do.