There could be a few possible reasons for Ichigo's escape aside from his power..could be that the spell wasn't complete yet as the 'thing' was still falling when Ichigo bust out and/or Tessai casting it sans chant lessened its power enough for Ichigo to make it out. Also, Tessai wasn't ruffled all that much when Ichigo escaped so that could be taken as nothing out-of-this-world happened..