Getting a rookie extension doesn't mean getting a max contract. In fact, a rookie extension is only required to be 25% of max for that player. Brown's offer won't be that low, but it won't be anything like the offers to Simmons or Murray.
The trend among Celtics players (Smart, Rozier) has been to turn down the extension offer and become a restricted free-agent.
Brown may want to sign an extension however, rather than become a restricted free-agent in 2021. There will be a lot of elite unrestricted free agents that summer. He might be better off signing now rather than waiting.