The model is probably based off of your watch history before you deleted everything. There would be no way (theoretically) for anyone to see your watch history

The model is probably based off of your watch history before you deleted everything. There would be no way (theoretically) for anyone to see your watch history but YouTube still thinks those old videos are ones that you'd be interested in and fortunately you haven't watched them yet (as far as the algorithm is concerned) so it's recommending them to you again.