This would help push back arriving at the repeater tax once younger players like Jaylen Brown and Jayson Tatum exit their rookie scale deals, and helping cost not become a significant limiter on the team's ceiling before these players truly hit their primes.
Curiously, Marks believes this might even be possible after bringing back Terry Rozier in free agency. Given his restricted status and perhaps smaller market in a very populated free agency market (as much as half the league may end up on the move, with far fewer cap than hoped for salaries), so it's not out of the question Scary Terry's market may prove more tepid than he may wish.
Horford coming back at a lower number ($20M range) with length (3 years) on the contract would allow Boston to have the full $9.2M midlevel and $3.6M bi-annual exception to use in free agency. C's are still under the tax if both are used and Rozier brought back.
Of course, this all would require coming to terms with Horford on an extension in that range, which is by no means guaranteed. But the prospect of securing a rosier (if not necessarily Rozier) future coupled with Al's seeming openness to such a deal hint this isn't such a radical proposition, either.