Yes, they do only do rush seats when they have seats to sell... and this is the first time I've not been able to get a rush ticket, so what does that tell us about this show relative to other shows?
There were also, as you note, less than 200 seats available in the entire 3,850 seat house as of an hour ago, which means - if my math is correct - they were at 99.5% capacity. How many shows hit that mark? When I've gone to The Met, I've seen many, many more empty seats. So, I don't entirely know what point you are trying to make by pulling a SummerTheater and obsessively counting available seats, but by any metric, this would seem to a show that is selling very well for them.
But sure, go on denying that this has been a successful run and throw up hypothetical road blocks for why this and other Black-sung operas are uncommon and, it seems from your general tone, should remain so.