W
Welder
That’s an absolute load of old tripe. I have soldered 100’s of thousands of fittings over my career and damp/ wet rag cooled soldered immediately after soldering.. the prelude to wiping the joints of flux. The key is to not just fly in with to much pressure!! I lightly dab the damp/ wet cloth on to pipe, until it solidifies fully and then hand wrap cloth to cool further and clean.
I have spent many years contracting and this is absolutely necessary to work at speed. I can hand on heart say, I’ve probably only had 20/30 soldered joint leaks over my career... and that’s probably due to rushing and not fluxing enough.
To be fair, you've just written it's not a load of tripe!
He specifically said "too quickly" and you agree by doing it slowly. You are in VIOLENT agreement 😉😕🙄