I don't think the word 'airtight' was appropriate. However, I have come across this several times before in different airports. It does say re-sealable, which implies that the bag must be capable of being sealed (rather than airtight). I understand that this is because they want any gasses from the contents to accumulate to enable their 'sniffer' machines to do their job.
Whilst it does seem a bit pedantic at the entrance to security there are free bags on offer for people to use. Really don't see the issue here. I always grab a couple just in case because in places like Manchester and Newcastle you would have to pay £1 for the priviledge of picking up a bag.