I'm assuming he means the witness data part of the transaction, which is where ordinals are stored. The witness data is required to verify the state of the chain, but can be discarded ("pruned") after verification, to save storage space, usually. A broad repudiation of ordinals' claim to being "permanent storage" could be achieved if node runners ran modified core software that pruned ordinals witness data.
Me, pretending to know what “prunable” means during nostr:npub1au23c73cpaq2whtazjf6cdrmvam6nkd4lg928nwmgl78374kn29sq9t53j‘s talk on Ordinals. 
Discussion
To be pedantic, pruning can happen even at non-witness level. If you set it in the bitcoin.conf, you can just reduce your stored timechain keeping just the last blocks (of course *after* you downloaded and validated all of them once). Pruning is just discarding what you don't need anymore after sync. In principle, everything but the UTXOset is prunable (assuming no reorgs). Witness program is somehow even "more prunable".