There have been some really good technical mudslinging threads about what constitutes a true full custom ASIC, but there is no question that more than one solution is really being produced. The chances of them all coming out if the same factory in China is not even remotely likely.
I think confusion comes from confusing chip etching and assembly. Quite possible chips are etched in same facility, perhaps even same wafers. Not sure if China includes other China (Taiwan) but is possible.
As for assemble units for sale, is very unlikely that same facility is making BFL, etc. boards. Many PCB assembly in China, Taiwan, Viet Nam, USA, everywhere. Not so many chip fabs.
Relationships much easier to establish with PCB assembly. Send tech packs, request quotes and samples, test, good going. Chip etching is much more expensive, so most relationships with intermediary trusted by fab to verify chip design so not to waste production run or fab time.