There is no inconsistency. Natural gas is most certainly flammable once it's vaporized (it would be a very poor fuel if it wasn't), but there's a big difference between something being flammable and something being explosive. If you have a big leaking tank, it is very likely that it will catch fire, and no one really wants a big raging fire in a residential area, but that fire will only burn at the leak where fuel and air are mixing. It's very difficult out in the open to form cloud of natural gas and air with a uniform mixture such that it can all burn at the same time, and even then the flame front is too slow to cause a damaging explosion.

I would also note that the engineer's use of informal language here is clearly alarmist.