Two more good points. Good catch by Ross and Becca. I think the glaze would definitely cut down on the ability of the clay to breathe as regular clay pots do. I also missed the part about no new growths...
So, the best way to assess the situation is to check out those roots and repot!
The glaze absolutely will keep the terra cotta from breathing. In fact, it will stop it altogether. The glaze is water-tight. Likely these will cause water retention at least as severe as a plastic pot.