How did you try to communicate that? I cannot find any indication one way or the other. Just showing it on a laptop and a phone is not good enough - maybe you have native apps on multiple platforms. Plus the phrase "there's an app for that" is almost always associated with native apps, not web apps.

I would specifically say something like "nothing to download or install!".

I mentioned "Understoodit runs on devices that you and your students already own: smartphones, tablets, netbooks and notebooks." But I agree I think saying 'nothing to download or install' is a great idea, thanks!