Yes, it's trivial to do. There are only 900 exit nodes. They could run every tracking request against that list of IP addresses, or a more comprehensive list of tens of thousands of known proxies and VPN end points, to flag suspicious packages. That's why it's surprising there's no concrete evidence of it. Then again, it's surprising that LE fails to do a lot of things, either through incompetence, lack of resources, or just not caring. More importantly, the threat is entirely avoidable: don't check tracking over Tor. So why would you want to test that?