Earlier quoted context omitted.
I've never looked at the source code of a PT, but my understanding is that they also implement heuristics to detect strange behaviors, i.e. clients trying to fake their ratio.
Not all of them do, but yes. Since it is the client who send all the info (bytes sent/received, version, etc), anything can be spoofed. The tracker, having access to the stats of all the client on a specific torrent can find "unusual" client (for example, a client that report a lot of bytes sent while no other client report downloading/receiving anything, or the opposite). But due do quirks of the networks, delay and…
For example there's also something called "ghost leeching" (side channel entirely bypassing tracker reporting) which can lead to other peers reporting upload for which there's no opposite account of download on the tracker. Making it look like peers over-reported upload and cheated when they are in fact entirely innocent. There's no way for a private tracker to be really sure about stats. The most the moderators can do is to check for repeating suspicious usage patterns across many torrents of a particular peer under scrutiny.