I just deleted an alt Google account thinking it would delete all the YouTube comments I made with it. Is this true?
According to an old reply (from 2011) on the Official YouTube forum, no, deleting an account will not delete the comments published with that account.
Try to recover your account in order to be able to delete the comments you posted with it.
https://myaccount.google.com/deleteyoutube tells you what happens if your YouTube account gets deleted:
The following will be permanently deleted:
ksubscriptions to other channels
msubscribers to your channel
- Comments that you made on YouTube
- Your replies and thumbs-up on comments, except those made on comments that came via Google+ [unless G+ gets fully deleted as well, which does happen if you fully delete your account]
- Your messages
- Your search and watch history
You will lose access to your purchased content: