You're right; I miscalculated the value of a minimum UDFA contract.
Roberts' salary for all three years is exactly the NFL minimum, so the $7,000 signing bonus is the only extra cash he's getting. His cap hit for each year is $2333.33 above the NFL minimum for that season, which is simply the prorated value of his signing bonus. And that's not even getting into the fact that the 2021 figure is bogus anyway, since the CBA expires after the 2020 season.
This may be a total coincidence, but I find it intriguing that the Packers are guaranteeing him the value of exactly nine weeks of 2019 rookie practice-squad salary. I'm not sure what, if anything, to make of that.