Based on what has happened with Samyang, Viltrox and Yongnuo, it seems Canon has been able to prevent them selling RF mount lenses with auto focus on the basis of intellectual property infringement. Whetherv anyone will work out a way to get around the intellectual property issues remains to be seen, but if not it looks like there will be third party RF lenses with AF only if Canon decides to grant a licence.
I don't think it's the RF *mount* that is the issue - otherwise Venus Optics (Laowa) would presumably also have been barred from producing their (Manual focus, non-electronic) RF lenses.
The issue, allegedly, is that the RF autofocus protocols would be impinged on. But, as
@Docofthewild suggests, it should still be possible for third parties to produce AF lenses using the EF protocols, as they've done for many years, and to simply extend the barrel and incorporate a RF mount. This would effectively be the same as using a third party EF lens via a EF-RF adaptor.
Equally, they could adapt their existing more compact MILC lens designs to fit the RF mount, but use EF protocols, ignoring the extra RF pins. That way, the AF protocols of RF would not be impinged on, and no RF intellectual property would be stolen.
It may simply be that Sigma and Tamron are uncertain of the legalities, and don't want to risk a damaging conflict with the Big Bad Giant, so they'd rather wait until Canon gives their "kind permission".
Meanwhile, they can design a whole range of lenses for Sony and L mount, knowing that if/when Canon releases the protocols and grants licences, they can easily adapt those lenses by updating their firmware and wiring to the appropriate RF pins.
