You can actually discover those in open weight artifacts, reproduce them, study them and issue a security bulletin.

With proprietary hosted weights you can be specifically targeted and you would not be able to reproduce nor prove anything.

Poisoning open models would be of short-term benefit to China only if they could target US (and maybe EU + Commonwealth) specifically. Damaging anyone else would be a net loss and would erode the partnerships and alliances they are trying to build elsewhere. So it's a fire-once weapon with a huge risk of collateral damage.

Much more plausible is simply making the models ideologically biased, but as history teaches us, preferring ideology or religion over science is a well-known path to ruin. It would be weird to simultaneously warn public not to use their own open models, so.

I think the most plausible explanation for open models is simply that Huawei wants more customers and is willing to compete on the hardware front.