If you're committed to Agile, I don't understand why you're talking about individual commitments at all.
The team commits to sprint scope. The team succeeds or fails in meeting that commitment as a group.
Beyond that the team should be free to figure out how the commitment is met.
We have one team that works 10:30 to 6:30. Another that works more traditional 9-5 hours. One is largely remote. Another is co-located. They all produce high quality work at a solid, and most importantly consistent, rate.
If we tried to force some work pattern on them, I doubt they'd be as successful.
https://www.joelonsoftware.com/2007/10/26/evidence-based-sch...
This is the crux. How does the business decide what's a reasonable amount of work to get done in a sprint? If the dev in the OP has time to work on his own stuff, why shouldn't the business ask for a bigger commitment in the next sprint?
Of course, there's meetings and fires, and so the dev should be available for that, but, in all, who cares about the number of hours?