I do not understand the xz format enough to evaluate that claim myself, but TFA explicitly claims that garbage data is a problem.

Hum, no. When you tar a set of files directly into a tape, you don't know the resulting tar size beforehand. Even less if you compress the result.

I would think you could just write the size at the end of the tape?

When you're reading the data later, how would you know where to find "the end" if you don't already know the length?

You don't know the end of the data, but presumably(?) you know the end of the tape.

We are talking past each other here...

Making a backup with tar is done by typing something like that on bash:

> tar -c - dir1 dir2 dir3 > /dev/tape

That will (hopefully, I doubt I got the tar switches right) backup those dirs into the tape (that will actually have a weird name, not '/dev/tape').

Now, in practice Linux doesn't always know the size of a tape you inserted. But this is not the issue, if you accept the seeks needed for that, you'd better write at the beginning anyway.