Other activities that have the exact same effect (a conversation being queued, if you switch to the sim not engaged in the activity, for them the conversation is already happening, the sim not doing the activity stands around watching the sim doing the activity with conversation idles until the activity is done): Repairing items and building treehouses.
Sims are also unable to talk while playing Don't Wake The Llama. They talk during all other board games (chess, simbles, cards, puzzles). But it's different from the knitting or repairs in such that other sims will immediately stop (or don't even start) trying to talk to the sim doing the activity.
But treehouse building and repairs is exact same bug as knitting. I assume it is a bug, because it sure seems like a conversation is supposed to happen but doesn't work for some reason.