Is this fix for a cracked whole house fan collar sufficient?
As cheesy as the tape looks, it will do ok, It is made to resist the temperatures that blow through the duct work. At worst it may fatigue over time, but I think it will remain intact. To be certain, later when it is running check it out up in the attic. If the plastic shroud that has the tape on it is jiggling, the tape may fatigue. Aluminum can't take a lot of movement.

The tech is possibly quite right in their remark about the part not being available.

If there about an 1/8" of room between the shroud and the blade, it will do well over time. Should it ever strike it, it will be in the near future, within weeks, if not sooner if it decides to touch again. Then the fix will be a heat gun to get the plastic to stay.

I really think that it will be fine. What I describe would be worst case scenarios