Not sure I am the best to answer, but here's a go :
You are correct about the project being rewriting the games in html5. However this is an ideal, and it requires competences, workforce, and time we do not have. The biggest and more requested projects (Hordes, Mush, Dinoparc, ...) are in the works, but smaller games still miss interested developpers.
Since the current Twinoid games will work as long as MT's servers remain up and running, a part of our team has developped an app to access them despite the official end of Flash. This is a provisional way to play the games, and we are aware it is far from great. Hopefully we will not have to continue with this method.
Now, regarding the security issues, I honestly miss the competences to give an answer. But, as I understood it, the app is limited to Motion Twin (and Etwin) games only, which means it is not more dangerous to use it than to simply play the games in your regular browser. If any security issue arise, they would come from MT's servers themselves.