I also don't understand why comments are not turned on in the app. It doesn't make any sense that I have to log in through MP.com via a web browser to do so.
This just seems like bad UX to me.
If there is a concern about comment SPAM, then consider limiting comments via the app to users that have had a profile for a certain amount of time (e.g., a year+) or have commented on other routes X number of times or limit the number of comments that a user can make via the app to X per month.