I think the key points are in section 2.3 of the terms (https://zed.dev/terms). "Certain versions" of the editor are open source.

So the source on GitHub looks to be available under the licences you've spotted, but perhaps the downloadable binaries from the main website contain additional code that isn't open source.

Any proposition is vacuously true following the contradiction between GPL and commercial software. As for your legal exposure, no one, not even Microsoft, has the resources to pursue your infringement, imagined or otherwise.