It makes total sense when you think about it. Even if Hot Toys, at one time, had a worldwide exclusive license for a given property, that license wouldn't be forever. They wouldn't be able to just sit on it in perpetuity. While it's possible they just continuously outbid other companies for such a license every time the term expires, therefore "locking it down," ultimately it is at the discretion of the IP holder who they sell a license to and what the terms are (such as length and exclusivity). So if HT has truly been sitting on a worldwide exclusive license for Terminator for years without creating any product, it would seem the IP holders are the ones to blame for issuing such a license.
At the end of the day, the "HT buys up licenses to prevent other companies from having them" thing is pure speculation.