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It works across multiple platforms, so it is cros-platform. Perhaps the term would be platform-universal or something?

It's a fair use of the term, and the kind of thing I would expect from old Microsoft. Maybe they'll add support for mac, linux, bsd, etc later.

It's an Open Source project so maybe community members will add support for those other platforms?

Quick search of the project issues shows that macOS is already mostly supported: https://github.com/Microsoft/reactxp/issues/19

It looks the project leads want more consensus on Linux React Native (ie, something actively maintained and preferably merged to mainline React Native support): https://github.com/Microsoft/reactxp/issues/41

Then again, different platforms have different sets of hardware features, so sharing 100% of the code is usually not possible or desirable if you want to make the most of each platform.

I think the emphasis was on "web, iOS, Android, and Windows" (no macOS or Linux), not "most of your code".

Just two would qualify it as cross platform.