Could the Meta NDA be used as a trojan horse? Anything Meta discloses to a developer/admin could be legally used against them if they acted upon the information. Not just leak of information shared, but modifications to Mastodon or Pixelfed that go against Meta's pursuit of success in the Fediverse.
If you are offered a meeting with Meta and are required to sign an NDA, be sure to read it fully so that you understand what your obligations are and what freedoms and friends you give up.
Just don't give up any of your freedom, never.