I was only successful pairing the GRILLPLATS outlet using the Hubitat Mobile App.
Just running through some tests here. I can now confirm your findings, @HAL9000. When using the Hubitat Mobile App on my iPhone, to pair Matter over Thread (MoT) devices, it does indeed appear to use Apple Home's Thread network instead of the Hubitat C8 Pro Thread hub's Thread radio network. Easy way to determine this is by looking at the IPv6 address of the MoT devices on the HE hub's "Matter" settings page. Everything paired directly to the HE hub using the Hub's web user interface has the same 'fde1' prefix. Only the GRILLPLATS outlet is different.
Just before this image was taken, I also had a TIMMERFLOTTE Temp/Humid sensor showing the same prefix as the GRILLPLATS outlet. That TIMMERFLOTTE sensor was originally paired to the C8ProT hub using the web interface, then later via the Hubitat Mobile App, and then just now using the Hub's web interface. As you can see, its IPv6 prefix is now the same as the other directly paired MoT devices.
Thus, this leads me to the hypothesis that using the Hubitat Mobile App on an iPhone/iPad, assuming Apple Home is set up and already has a Thread network, that all MoT devices will use the Apple Thread network instead of the Hubitat Thread network. If this hypothesis is proven to be true, then for sure the Hubitat Engineering Team will need to figure out a solution.