Yeah, that's a much simpler explanation. The data any given smart device sends (unless it's streaming content) should be on the order of a few megabytes per day, if that. All it would need is:
Depends on what it's sending. Something like a washing machine would only need a few dozen kb/day, but something like a temperature sensor could send MBs, depending on update freq.
Regardless, I want to fully control any smart device in my home, which is why I tend to not buy smart stuff.