No I don’t agree with him.
As I said, I don't think many people would.
The junction may start out as “accessible”. Back in the day, many houses had a couple large central octopus junction boxes where all of the lighting came together.
In those cases, one might have been able to make a reasonable guess as to where those JBs might be, but when (as 'usual') it's a matter of individual 3/4-way JBs, they would, if under floorboards, rarely have been 'accessible' (to my thinking) even at the start - so one doesn't really need to speculate about how much 'less accessible' they may become in the future (which, as has been said, one really can't predict).
So,
IF one feels (as the regs do) that screw-terminal JB's must be 'accessible for inspection and testing', then my personal view is that they should then be "really" accessible (in a fairly everyday sense of the word).
However, speaking 'personally', there's a fairly big IF in what I've just written. My experience is obviously very limited, but I've personally never encountered, or even been aware of from 'contacts', a problem resulting from an
undisturbed non-accessible (e.g. under floorboards) screwed joints, even when they have been in service ('undisturbed' throughout) for decades. It's with recently-installed screwed joints, or ones which have been 'disturbed' in the name of I&T (or 'maintenence') that I have sometimes seen issues.
Nevertheless, the regs are the regs, which constrains what we can say/advise.
Kind Regards, John