Right now IsNumber(ToNumber(0.1)) returns false. I think is should work with decimals too.
I dont see it as false - can you share your document with support@coda.io to take a look?
Here is my doc, and screenshot:
But I think you will see true on your machine. Because I see true here in embed (but false in doc). So, it is something with locale decimal separator, I suppose. Formulas are calculating client-side, so my Chrome’s JavaScript expects comma because system’s locale set to Russian.