Yes, that's true, but I suspect that its really a 'swings and roundabouts' situation. Whilst flex is less likely to snag (both because of its greater flexibility and its round cross-section), I don't think it's outer sheath is as tough as that of T+E.I have found twin & earth seems to snag when drawn through conduit. I would be temped to use a flex as the smoother surface does not snag as much.
I rather suspect that there's some theoretical scaremongering at work here. I seriously doubt that washing up liquid, soap or even tallow represent a significant explosion risk. However, as you say, there are some products 'made for the job', so I suppose it makes sense to use them!I have in the past used washing up liquid until I was told how it contained benzene and could explode. Now I use Yellow77 designed for the job. Also in the past used soap and tallow not sure if they will explode but not inclined to do the tests.
Kind Regards, John