I think you're down the right alley. The prototype builds that were mentioned earlier are going to the German government. They're heavy and have odd wiring that'll make them problematic in a commercial fleet.
Both Hainan Airlines and Hong Kong Airlines (Hong Kong Airlines is a subsidiary of Hainan Airlines) are having financial difficulties and lessors (particularly AerCap, formerly ILFC) are not allowing deliveries of aircraft due to non-payment. There are a few A350s that have been stored in France for several months.
For all SAA's problems they've always made lease payments and have a very good relationship with AerCap (the leased A340s are leased from AerCap). I suspect these are the aircraft in question and that AerCap have placed them at SAA. Both airlines have their A350s in a C33Y301 configuration which won't be suitable for SAA. They'll want a bigger business cabin and likely something more consistent with the A330-300. AerCap will have to have these cabin mods done first and this will take some time.
As an aside, the Bank of China areone of the world's largest lessors. They have about 500 aircraft in service or on order and offer similar terms and business model to any other lessor. While they do lease to Chinese customers, most of their customers are abroad and include Air Canada, Southwest Airlines, IAG, ANZ and Qantas. Nothing "rescue" about them. Also, contrary to the name, while they're now Chinese owned, they were originally Singaporian.