He is right in economic sense, but adding inflation would be a precedent for more changes in the future. It would break the idea of predetermined scarcity. Bitcoin is scarce, not because there are only 21 million of them, but because the rule which enforces the limit is unchangeable.

Incentives make this thing work. Incentive to protect the 21 million supply must be larger than the incentive to change rules and add inflation. If that can be guaranteed, then unchangeability is guaranteed.