Geek Bike Family

The theory goes that you can't actually seal a tube, there will always be micro holes somewhere to allow moisture, not water in.

Yeah, we aren't talking a tube filling up with water from being ridden through a river etc, small amounts of water vapour, moisture from the air, etc get in (temperature and pressure changes can effectively 'suck' moist air into a tube through a very small aperture) and this can, in my experience*, accumulate enough to be sloshing around inside a 'sealed' tube.

*Yes, I've cut up more than one frame that had no breathers and had ancient, stinking, rusty liquid pour out of the tubes.

I'm not saying every frame without breathers is destined to rust but my experience is that non breathered frames are generally in a worse state than those with breathers.

And I do find it quite ironic @TvH that you are pointing us to videos of someone who doesn't put breathers in his frames talking about having to fix rusty tubes. The video where there's a hole in the back of a seattube, in my opinion probably wouldn't have happened if the seattube was vented into the bb shell. He is also talking about powdercoat though which is a treatment I feel has no place on a bicycle but that's another conversation.

In the case of Vaz, if he is brazing and not soaking I find that concerning as while you can blast the outside of the tube fine, the only way (I know of) to get flux out of the inside of a tube is to soak it out. Maybe he's using a gas fluxer and that might mean that there is no flux inside the tube, I'm not too sure how that works.