I don't think such a contract is even legal in the NFL since teams cannot block coordinators from seeking promotions. However, ever since going to Penn State, BOB has developed a personal "rule" that he's stuck to and made widely known that he makes two-year commitments. He stayed at PSU for 2 years even though he was offered HC jobs in the NFL after 1 year. He was at Houston for a while. Then at Alabama, he probably could've left in 2022 for an NFL coordinator job somewhere (NE, most likely) but stayed, and Belichick likely was honoring that as well. So I kind of assume that him signing here also carries a two-year commitment, even if it's not on paper.