Man that foreman nailed it. Think about it, if the fitter doesn't get those seams tight and the gaps perfect, even the best welder is just filling a bad joint. A sloppy fit forces too much filler metal, which can cause weak spots and stress inside the weld. The welder's torch follows the line the fitter gives them. A perfect fit is half the battle for holding pressure.
Sounds like that foreman has seen some real disasters. Imagine a fitter leaving a gap you could drive a truck through, then the welder just piling on metal to fill it. You'd end up with a weld that looks like a bad scar and probably leaks under the first bit of pressure. It's like trying to fix a crumbling foundation with a fresh coat of paint. The welder gets all the glory, but the fitter is the one who sets the stage for success or failure.