In fairness, I think it's one of those things people go batshit about pretty unecessarily.
How big an explosion can you get from an empty motorcycle fuel tank with the lid off if it goes off bang? I mean yes, it could make a fair old bang and give you a goodly jet of flame up out of the filler cap if the vapour mixture gets exactly right but is the tank itself going to grenade? Can't see it being any worse than when we used to deliberately explode the exhausts of our 2-strokes while decoking them by pouring petrol in, lighting it then blowing an airline down them.
If you're welding, you ought to be wearing a minimum level of PPE so a flash explosion isn't going to hurt anything but your underpants and if you aren't prepared for stuff to catch fire when you're welding, you're in for a surprise.
I have an old petrol tank somewhere. Also have an ignition coil and sparkplug. I might deliberately blow it up sometime just to see what happens and post the video on youtube.
Probably the safest way would be to pour in some petrol and set it on fire then weld it while it's on fire
I reckon it's mostly that welders simply don't like welding petrol tanks because they are usually thin and far rustier than they look. The explosion risk is a good excuse not to. If I had to fix one of those tabs, I'd probably go with braze or silver solder over welding anyway. Much less chance of making more holes than I started out with.