Yes, Edward Snowden had a few public announcements in the past that were true, so why shouldn't this one be? On the other side, any application can be used for surveillance if designed that way. Google is already using different applications to follow people's activities on the internet so it could target us with those annoying ads.
Yes, Google developed Allo for mass surveillance. Originally is was supposed to be an encrypted way to chat, with the logs being ambiguously stored so they couldn't be subpoenaed and end to end encryption. Google then backed off of all the privacy features it originally touted, opening it up for mass surveillance.
I don't know that Google Allo was designed for mass surveillance, but if Snowden says people should not use it, I believe that it is quite possible that it can and will be used that way. The technology is out there, and as long as governments or corporations can use it that way, they will.
Google did not develop Allo for the purpose of mass surveillance. Edward Snowden is not an expert on Google products. However, users of Allo should take some security precautions to protect sensitive information from hackers. There is a greater threat that a hacker might steal data from Allo, then it being used as a mass surveillance program.