Wouldn't licensing under GPL limit their flexibility with respect to external contributions? They wouldn't be able to use that in proprietary product, or move the project's development back to a closed model.
Depends. Some companies use GPL with copyright assignment to control the project: you can use it as a wholly open source product, but the company has the right to do future development in a closed source way, embed it in proprietary products, etc.
I don't know if that's likely with a build tool, but it happens elsewhere.
I don't know if that's likely with a build tool, but it happens elsewhere.