I am sure that a large amount of confusion is simple miscommunication, but I thought I'd share my experiences about a year ago before I first got the flow hive.
It was spring time in Perth. I had read a couple of books, been on this forum and was excited when the Marri flowering happened, so decided to buy a brood box to act as a lure hive to try and catch a swarm before my flow arrived (first orders took a while.....).
I went into a beekeeping supplies shop in Perth and chatted to a lovely lady about my plans. She sold me a bottom board with box attached, lid & frames and advised me that I should only put in some frames to leave a gap for the swarm to fit in.
A few days later the flow hive arrived :), so I took back the box to swap it out for a loose bottom board and box, as that was what was with the flow hive & would give me more versatility to mix everything up. This time I met the male owner of the shop and I am going to give him the benefit of the doubly that he was having a bad day!
The conversation deteriorated rather quickly when I tried to ask him if he sold screen bottom boards. He said all my bees will die with a SBB. If I were having this conversation one year on (and he wasn't so aggressive and shouty), I would have probably tried to explainthat indeed there is the core flute that we can slide in to cover the bottom. So whilst my bees have not died with a SBB, I essentially run it like a solid bottom board anyway so maybe he wasn't totally wrong!
He then said not using wax foundation was "stupid" and would fail. I know now that this is also not a cut and dry issue, and we indeed do use mostly wax foundation.
Then comes the bit I have the problem with. Trying to get the horrible conversation back on track I tried to run through my swarm catching plan with him. When I mentioned only putting half the frames to leave a gap for the swarm to measure out, He said, and I quote "what idiot told you to do that, that is a stupid idea". At this point I didn't tell him that that was his wife, nor did I try and argue that she was indeed absolutely correct, based on everything I know now.
I decided to give up on trying to talk to this man and left.
I think that understanding and explanation are the key on both sides. Very often there are many ways to go about doing aspects of beekeeping, but sometimes some things do not work in a local climate (such as trying to establish a Nuc in 2•C). My beekeeper at that time was not in the mood to have a discussion, or explain any of his reasonings, even though I believe I asked for explanations respectfully and did not disagree with his rantings at all.
That said, the beekeeper in question in this thread is giving his time to mentor others so I'm guessing likes to explain how he goes about his work. I would simply politely ask for an explanation at to why he won't sell the Nuc. At @RBK says, it may simply be the wrong time of the year. It may be the size of the box (8 frame seem very common here in Perth so wouldn't cause a stir here). Another option would be to simply ask him what he wants you to do to set up the Nuc and then see if he would be happy with multiple 8 frames to make up the space.
My gut says it is a miscommunication issue, rather than a grumpy old Beek issue. Indeed my grumpy old beek is now selling boxes for flow hives, so may have changed his attitude to flow once he actually saw it. Either that or he saw a good commercial opportunity - i won't go back to ask
Sorry for the ramblings,