It's not unknown for perfectly serviceable 744s vent fuel from the surge tanks in the wing tip area whilst on the ground, stationary or whilst taxiing. You presumably had somewhat more than normal so the flight crew decided to return to the ramp to get it checked out. Having the fire services involved is routine for any fuel spill and the flight crew would certainly follow their advice in their handling of the situation - as others have said, a puddle of jet fuel is not "that" flammable.
There are certainly company Standard Operating Procedures for the handling spills but I'd hazard a guess
that at the airline in question they don't
demand undoing of seatbelts and/or an immediate disembarkation. It's also worth bearing in mind that once the belts are undone there's a tendency for some/many passengers to get up, go and chat to their mates, get bags out, block the aisles, etc etc so maybe keeping the belts on was done on this occasion to prevent "crowd control" issues.
a member of the crew said that passengers should not be onboard for and also due to the size of the fuel leak that everyone should be off the aircraft as well.
That sort of comment might earn a
for the crew member if it got back to the office....At the risk of igniting debate ( pun intended) if you heard this from a member of the cabin crew then it's worth being aware that most of them don't have any significant technical training, since it's not knowledge they need for their job. Also they certainly wouldn't have been privy to the information the flight crew (i.e. pilots) would have been getting from the engineers and fire crew..... i.e. the real experts, may well have recommended keeping you on the aircraft.
Purging the fuel tanks? No idea.