Only 2 messages ever appeared in the disposable email address in-box and both were from Cocoon.
The first was a "Welcome to Cocoon" message. The second from them was a message about fixing a problem some users were having with Cocoon. (I didn't experience that particular problem.)
So the results are, at best, inconclusive because I don't know if the ISP hosting the disposable email account was flitering out any spam before it landed in the in-box.
Since the initial torrent of spam to my personal address, there's been no more. To be clear, I haven't logged in with that address since the spam appeared. Curiously, I never received their email about fixing that problem at my personal email address. Don't know what that means.
I'll relay my findings to Cocoon and the disposable email account can be closed.