"A few years ago Chiefpilot told us this precaution is no longer necessary, as the modern equipment is totally different, no more valves (?), and so on, and a big debate arose."
Peak output power still remains roughly the same for HF USB transmissions, LEM (400Watts) on modern HF systems. It shouldn't make a difference how that power is generated (valves or transistors).
Proximity of wiring and antennae to fuelling systems may have some bearing on the issue (many Boeing aircraft now have the HF Antennae/Tuners in the tail) rather than in the wing/wingtips. However, although I've found the following hard to believe, I've been told that transmissions can cause pieces of lockwire in remote locations to heat up at resonant frequencies (Perhaps if this lockwire is exposed to fuel dripping from refuelling connectors you could have a problem).
I can't say I've never accidently tuned the HF during refuelling, but it's not something I would make a habit of intentionally doing.
Perhaps your Chiefpilot is thinking of Radar technology where the power output is a lot less?
Regards.
Q.