What do you mean with third-party-service?
Your software is a "third-party service" (But so is Steemit, Busy and any other front-end, since the blockchain cannot be accessed without the use of such implementations).
You got to have them saved somewhere - otherwise how do you use them?
I think that by this he meant easily accessible and always "active" (like when you use your browser's password manager to automatically input your password regardless of the user, and using cookies to keep the password wif logged in your session).
But with proper safety measures, this messenger would not be a security risk, I believe, and would instead make it much faster and safer to send and read transactions (especially if real-time notifs were enabled in the future).