I think that having a formal spec would help since a lot of the minutiae that is done to format and the like can be in the form of snippets in (nearly all) text editors used today.
So just having the autocompletion for people editing in text does wonders.
As for standardization, applications don't have to target it as their on-disk format for it to be useful. Instead of bespoke JSON/XML schemas or proprietary formats, having a plain text export format is a big win. Specially if it's human readable and human writeable.
I'm not sure this is the best tool for an import-export format, as it is unable to capture all of the data available in some to do apps.
I could see it being OK as a sort-of data backup format. But it's missing a lot of data that really ought to be tracked by an app, like creation, update, and completion times for each item. Many apps also track priority, sort order, hidden/unhidden status, attachments, and other things. This stores none of that.
Even then, the value is fairly small. In 20+ years of using to do apps, I can think of very few times that I've referred back to completed to do items.
So just having the autocompletion for people editing in text does wonders.
As for standardization, applications don't have to target it as their on-disk format for it to be useful. Instead of bespoke JSON/XML schemas or proprietary formats, having a plain text export format is a big win. Specially if it's human readable and human writeable.