It is, if that's what it was, but it wasn't. Bleach and ammonia make chloramine gas, not mustard gas; the latter requires additional components that I won't list here.

Chloramine gas is still dangerous until it's dealt with and the space is fully ventilated, but not to the power and severity and immediate danger that mustard gas has.

That said, again, as a PSA to everyone: do not mix bleach with anything else other than water.