These sites (at least TeslaFi, that's the only one I've used) do not extrapolate data or forecast battery degradation, nor do they draw explicit conclusions that there is a problem with the battery. It simply logs historical data and renders reports on it, straight from the car itself. It does correleate that data with other events and data that an individual user would not have access to or think to check, such as software versions and other cars of similar vintage and mileage. This lets the owner draw his or her own conclusions about whether they think something is out of the ordinary.
Everyone (should) know that batteries degrade over time, and in general the curve flattens out after the first year. However, there have already been cases where software updates have reduced range or capacity via voltage. This shows up pretty obviously when you're constantly logging the car's stats.
There are a few chicken littles out there that will think the sky/battery capacity is falling, but a lot of other people just want to know what their car is doing. I learned just how different the cold months are than the warm months, what a headwind can do to your range, and how to better plan long trips by knowing what the car will do.
There should be little need for people to be watchdogs of their own car's health, but when you have OTA updates, you need to be able to defend yourself with hard data and not "uhh, I don't think it used to be like this" if the car starts behaving differently after installation. This helps keep Tesla honest, as they'll have a much harder time gaslighting the owner by saying "looks normal to me" when your car is measurably different from others.
The battery degredation reports on teslafi are extrapolations / interpretations of data from the car. if you email teslafi and ask them what their "degradation" reports represent, they will tell you its interpretations of data from the car. Its the "renders reports on it" part that is the issue.