The major difference here is that they are not making a directly competing product to Go, Linux, etc. I find the word "leeching" appropriate because here, it's not simply someone building on Terraform to sell something else, they are selling a direct competitor to hashicorp's products.
In another era "building on X to sell something else" would be still considered "embrace, extend, extinguish" (in the actual, originally intended sense), depending on the market power of the players involved.
At least if the "something else" isn't free software.
