The person I spoke with in the shop has a Brompton and frequently tours on conventional bikes. His colleague, who also has a B and tours, backed up what he said.
Neither said that you can't tour on a small wheeled bike, just that long days take a bit more effort
That's fair enough. I give more weight to advice that matches my own plans and uses.
The thing for me about using a folding bike for touring is that it would probably mean a substantial change in my style of touring.
I'd travel lighter, for sure

, but it would open up opportunities that currently don't exist or are too much hassle. For instance, plane travel becomes a whole lot easier (and probably cheaper) but the real game changer is the ability to hop on a train (or bus) with minimum fuss. It opens up all kinds of possibilities that I either don't have with a "normal" bike or that are just too much hassle.
I was very fortunate when I lived in NL to be able to use an excellent train network as a fallback - it was a real "comfort blanket" and was one less reason not to go off for a weekend. Here, in Spain, trains are quite bike friendly too - when there is a train

My impression, perhaps wrong, of the UK is of a very confused and confusing train system that requires reservations (often multiples for one journey, it seems). A bike that I could fold up and carry on along with my gear would open up so much for exploration.
It's a gamechanger.