That response and reasoning appears completely reasonable, and I don't see any suggestion it has anything to do with flow hive ownership. You are going to get this answer regardless of hive type.
This is the reality of the situation, nucs are ordered well in advance, established beekeepers know this and have ordered accordingly so are well ahead of you in the queue. Beekeepers I work with were taking orders in February for nucs that are only just starting to build up now (getting access to queens this early is still posing problematic).
As for the $500 nuc comment, I'd probably say the same thing to you today if I was pressed to provide a nuc on short notice. To meet the demand I'd more than likely sell you an overwintered hive of my own and expect to be compensated for the inconvenience.
Your profile says you have 10 hives, are they all Flow hives? If you have other hives, why aren't you splitting them? It's the perfect time of year to be doing this.