Interesting. It seems this carve-out was added when they rewrote the rules in 2018. In the original rules [0], it says:

   Before consideration, a proposed solution must be published in a refereed mathematics journal of world-wide repute, and it must also have general acceptance in the mathematics community two years after that publication. Following this two-year waiting period, the [Clay Mathematics Institute] will decide whether a solution merits detailed consideration.

[0] https://web.archive.org/web/20000622023328/http://www.clayma...