I Don't think hoarding is necessarily the right word. They were using them to research potential new products or tools. The theory being that if only a few of the projects prove high value then it's worth it. That's not hoarding that's letting them flourish.
1) in the case of Area 120, this is one of the ways it was pitched to management. “Passionate entrepreneurs are leaving to work on new ideas; if you give them a place inside Google to pursue new ideas, it keeps them and their entrepreneurial energy at the company.”
2) in general, early Google used to hoard talent all the time. The founders would keep great people (or their friends) on payroll for ~ever just to have them stick around. That was most prevalent in the first decade of Google’s life, to my knowledge, and mostly applied to very senior people.
By the time Area 120 was pitched and approved (circa 2014), those days were largely gone. Area 120 was primarily filled with junior people (L4-L6) and constantly had to sing for its supper - it was not at all a sinecure.
So basically google had a shed where they hoarded talented people, to prevent competition? :)