Why? You'll never get it all out and it's the fumes that are explosive (I don't know about diesel). I have an old can of diesel I use to start my fire pit and that stuff is hardly what I'd call volatile. Gas on the other hand, "who needs eyebrows?".
I can tell you that is complete bullshit, for a number of reasons. But I do believe that's the sort of story people like to tell.
He's welding on a couple small brackets on quarter inch, he doesn't need to penetrate too deep. I've quenched plenty of things in oil, I doubt diesel would explode.
I'm not saying it would. It's just not something I would ever do. Especially for an application like that, especially when there's an epoxy available that will get it done.
We used to weld on aluminum fuel tanks all of the time. 1/2" aluminum full of diesel. Or hydraulic fluid, but rarely gas as we didn't build boats that used integral gasoline fuel tanks. But all day long on diesel tanks. Surface welds, not structural, of course.
I'm with this guy but also filling the tank creates a massive heat sink out of it. I've welded the filler necks on aluminum diesel tanks before with no worries.
Surely you have an opinion on the subject? Spill it! I, no, we need some entertainment this early in the morning.