Its irrelevant as you need to leave ASAP abut this
Making lessons go flying in convective SIGMETs and reprimanding instructors when they resist
is not necessarily unsafe but needs to be looked at in context.
A convective SIGMET is issued if a certain set of conditions exist in a certain geographical area. Depending on the conditions (isolated Mega cell or embedded multiples) and which way they are moving it may or may not be perfectly safe to fly.
All of my instructional experience is in Florida and you pretty much always fly with a SIGMET during the summer time.
However all that is irrelevant as you need to leave for all the other reasons.
If you feel comfortable tell the owner why you’re leaving.
File a complaint with the FAA also.