Like many things discussed on forums like this, let's not confuse our perception of a WTS scenario with how the WTS and the faithful see it.
As WiFiBandit says, the WT has never termed it "bunker", it's simply a representation of how the WTS speculate things could be if Witnesses are under ban or otherwise restricted. TBH, meeting discretely and in secret has been a feature of their work in a number of lands over the years where Witnesses have been persecuted - this simply is set in a US house basement bringing it home to us softies in the West that this is what could happen.
Of course, it's fear mongering and playing up to the narrative they have around Armageddon etc. but it's not really much more than them showing how groups would have some kind of designated place to meet in the event of a severe crackdown and be under the responsibility of their group overseer.