Yes that is an old wives tale. It is impossible to swallow your tongue. Its one of those old wives tales that has been around for ages and causes well-meaning people to do ill-advised things.
Yes, it is impossible to actually swollow your tongue, but it can choke you,
as my boss and friend died because he had an epileptic fit and the term the paramedics used was
"he died because he swollowed his tongue"
and one of my close friends suffers for epilepsy, luckily she had a fit while she was at
home when i was there, but i was lucky to have her father in the house too,
he showed me what to do,
he did not put his fingers in her mouth because he said that she could bite them,
but what he did was move everything out of the way
and turned her onto her side like in the recovery possition,
because he said "there's less chance of her choking on her tongue"
and then left her to finish fitting,
the paramedics were called and they said he obviously knows what he's doing
as she had been a sufferer since she was a child..
you do not hold down their limbs or put your hands anywhere near their mouth,
just turn them on there side, make sure their head
is supported then leave them to stop fitting.