R2020b pre-releases is already out, and was out before the Apple announcement. In order for Mathworks to support the new system at the time that R2020b comes out, Mathworks would already have had to build in support in R2020b pre-release, and would have had to have released that support before the Apple announcement. That is not something that Apple would have permitted.
As far as Apple is concerned, "sometime in the fall" means any day before the Winter solstice near the end of December, and when Apple says "fall", they do not mean "before the autumn equinox" in late September. Mathworks release cycle is traditionally before the autumn equinox for the *b releases. It is rather unlikely that upon release, R2020b would support a computer that would not even have been released yet.
Without even looking at the specifications, just by knowing the history of the two companies involved, we can be almost certain that when R2020b is officially released, it will not include support for the new systems.
The more realistic question is whether R2021a will have support for the computer. R2021a pre-release will be sent out at close to the time-frame we can expect for the new Apple hardware to be released.
As far as I can gather, Apple does not consider Mathworks to be a big enough customer that Mathworks is likely to get access to pre-released Apple hardware. I could be wrong, but historically Mathworks has not been able to influence Apple much. I am not confident that R2021a will have support for the device, whereas I am confident that R2020b will not have support for the device.