Does this mean a developer can use TypeScript when using Angularjs (and specifically Angularjs 3)?
To Answer this you must understand the versioning of Angular 2 vs Angular 1
Angular 3 is the next release of Angular 2. Unlike Angular 1 to Angular 2 Angular 3 is like Angular 2.1 if you go by how Angular 1's versioning style.
This is a great Blog post to explain this. https://toddmotto.com/please-stop-worrying-about-angular-3#real-versioning
With this in mind Angular 3 will be like Angular 2 Google will continue writing in TypeScript and pushing the use of TypeScript but you can write Angular 2 and 3 in both.