when(platforms:) requires tools version 5.7+. on the aggregate i am seeing many package authors hike toolchain requirements to 5.7 recently, but i believe this is largely due to fallout from SE-0346 than a sudden disinterest in toolchain backcompat.
in theory it is possible to vend multiple manifests targeting different toolchain versions, but there is still no way for multiple package manifests to share definitions (as the manifest cannot have dependencies of its own.) so effective tools version is always much lower than the last minor toolchain release. (it looks like Embassy's effective tools version is 4.2.)
i personally am already hiking requirements across the board and would probably use when(platforms:) to gate Android-incompatible dependencies. but i have never really bothered to support more than 3 minor versions going backwards in the first place, and 4.2 to 5.7 is a much larger gap.
i don't see any indication that the package is particular about not having dependencies, only that it does not have any right now. it's perfectly possible that it would need to add one in the future, and that doing so could imply rolling back Android support.