It probably is in house. Remember Huawei led, and still leads the industry in 5G tech, they can't have nicked that IP from the west because.... The west admit they are behind, still, let alone 5 years ago. Chinese engineers do know what they are doing.
Loongson's uArch is pretty different to zen and skylake derivatives, chips and cheese have analysed their CPUs pretty closely and they are different enough to exclude IP theft. Stealing IP for an interconnect that can't work with your uArch is a bit stupid.
Of course the US would ban the CPUs on 'national security' grounds, but they happily let Cisco's swiss cheese equipment run core networks....
To be honest, another competitor in the desktop computing market would be great. It's a shame loongson uses it's own MIPS based ISA as mainstream support would be poor, mind you china is a massive market so maybe developers will release MIPS binaries just as they are now doing with ARM. I guess loongson could pick RISC-V but I assume their ISA is more performant or they would probably have switched given the significant software support for RISC-V. Either way, give it a few years and we may genuinely be picking between X86, ARM, RISC-V and LoongArch for new systems (in the actual free world.... The US will obviously stick to just X86 and ARM by banning RISC-V and LongArch)