Well, the example was actually one of the examples given by the author of the TinyGPS++ library, so I assume that something about the isUpdated function works with the gps.location (or speed, or date) object and only checks that portion of the string? But it actually seems to work fine.
Either way, I've written a quick little test code with the logic you suggested and will try it out on my drive to school tomorrow. I'll post an update afterwards. I do believe the problem I was having with that most recent code (latitude, longitude, etc. being equal to zero) is because I defined those variables on a different scope than the loops, and didn't print the change in those values within the scope of the if (isUpdated()) loop.