Maybe I'm misinterpreting the output from their backup tool, but isn't that exactly what it's doing?
Metrics for todoist-fullsync:
name 1 days ago 8 days ago
------------------------------
files 1 1
bytes 82363 86661
items 85 87
The "items" line there seems like it's actually parsing the file and counting the number of entries in it? It's also captured in point #2: "Can be intuitively evaluated as plausible or suspicious. If the number of tasks in my Todoist export dropped from dozens to 1, that would be cause for concern."