I don't think he left a lot of arbitration money on the table but he definitely left some FA money on the table (assuming there's no salary cap or decline in MLB revenues or other major CBA changes, which is another reason to do this).
The arb years portion of the contract is:
2019: $6M (let's include bonus, since that's also close to what he wanted in arb)
2020: $8M
2021: $11.75M
By comparison DeGrom got $4 million and $7.4 million in his equivalent seasons, before jumping to $17 million this year.
Gerrit Cole made $3.75 and $6.75 and just won $13.5 in arbitration.
So he's definitely giving up between $3 and 10 milllion in 2021, dependent on how good his 2020 is. But he's possibly getting a little extra these first two years.
It's definitely more about the $19.25 million guaranteed should he get hurt between now and 2022.