yeah, you should read the doc in the link, they explain why they couldn't use Onavo to simple man-in-the-middle snapchat users, hence the project to use the testing service provider to hire test subjects which would install a MITM solution to unencrypt snapchat (and later youtube and amazon).
Normal Onavo users were not subject to the decryption (although they were providing Meta information about overall snapchat's marketshare).
Which doc and which link are you referring to? This isn't anywhere in the class action discovery documents as far as I can tell.
TC reported back in 2019 on Facebook using various third-party testing services to distribute their first-party Facebook Research app, so I'm not even convinced that point is new (or that they paid the third parties to do any actual research).
Facebook would probably have run up against app store policies by trying to install their root cert on existing apps, which seems to be the most likely justification for installing this additional app through third-party channels. It's also a lot easier to avoid suspicion when it's part of a bunch of click-through screens as part of app setup (especially when you're dangling money in front of users, even just $20/month).
Normal Onavo users were not subject to the decryption (although they were providing Meta information about overall snapchat's marketshare).