Did you try force flashing again with the second command?
Regarding the extra price of 10GbE, it's not that much. Last time I read it was something like $19 for the AQC107 chip (2018 prices), which is 10GbE. Apple and big manufacturers like ASUS, Gigabyte, ASRock buy these in thousands so most likely costs cheaper to buy in bulk.
Apple ALWAYS charges a ton more for any non stock options, since back in the BTO days. Matter of fact if you look at their RAM and SSD prices, it's still ridiculously overpriced.
Another user brought up a good point about AQC111C inclusion, but I think it has more to do with deployment because afaik, 5GbE can work over old ethernet cables and 10GbE requires complete rewiring to CAT7. Maybe they were worried about market penetration that 10GbE is not ready for mainstream adoption yet. Which is weird because these chipsets can fallback to lower speeds like 5GbE if necessary.
And I don't think, as another user pointed out, it has anything to do with bus speed limitations. If you check my screenshot, it's running on a 4x PCIe lane (Most likely on the PCH), which is more than enough thoroughput for a full 10GbE. I've deployed a few 10GbE NICs on a few friends computers at 4x PCIe lanes and they are reaping the rewards at over 1GB/sec transfer speeds, which is pretty awesome.