NIL contracts are technically portable be rule, as they can’t be contingent on participation, let alone participation at a particular institution.
Also, these big contract numbers are likely based on total potential value, renewed yearly, with a larger sum at the back end. Something along the lines of, “We’ll give you $300k. Each of the next three years, you’ll sign a contract for $50k/year and, in the fourth year, you’ll sign a contract for $150k.”