It's far more likely that he was overly optimistic about Mailbox's chances of success at Dropbox. Founders are notoriously delusional about this kind of thing.

Dropbox wasn't worried about maintaining the small existing Mailbox userbase, they were betting they could grow it by millions of users. When it didn't work, it messed up their plans (and promises).

Never attribute to malice that which is adequately explained by stupidity.