This is by design. Ambient declarations are only allowed at the top level (see sections 11.1 and 12.1 in the spec).

But ambient declarations inside nonambient modules is allowed by both compilers, and we have been supporting it. According to @RyanCavanaugh and @danquirk there are real world customers who depend on this.

e.g.
https://github.com/BabylonJS/Babylon.js/blob/master/Babylon/Physics/babylon.physicsEngine.ts
https://github.com/BabylonJS/Babylon.js/blob/master/Babylon/Physics/Plugins/babylon.cannonJSPlugin.ts

It's also very useful when you want to directly use node.js's require without having to pollute the global scope with it (this is a very real problem when using node.d.ts with any of the compiler .d.ts files at the same time, for example).

Ok, got it. Will fix the spec.

This was updated in section 10.2, but still needs to be updated in the Grammar appendix.

Also, something else. The grammar was changed to remove AmbientExternalModuleDeclaration from AmbientDeclaration. However, the forbidding of nested ambient external modules appears to be in the semantic phase. So technically, the grammar should allow it by including AmbientExternalModuleDeclaration in AmbientDeclaration.

I'd prefer to keep AmbientExternalModuleDeclaration out of AmbientDeclaration so that the grammar reflects the restriction. The fact that the compiler verifies it in the semantic phase is really just an implementation detail.