It's far more likely to be Vault as a base, actually. The MPL would allow someone like Amazon to use Vault's source as a base, and so long as the core source wasn't modified Amazon would be under no obligation to make their modifications public.
The MPL is a lot more "business" friendly than the GPL.
The very fact that HashiCorp is changing their license and restricting use clearly indicates they see this threat as reality. Amazon/Microsoft/Google/Whoever using HashiCorp's work/effort but keeping all the money to themselves.