This isn’t really true anymore. Originally it was and because SMS was rarely used it was effectively free. But then it grew more popular to the point where most messages didn’t have “unused bandwidth” to piggyback on and had to be sent separately. Now days all traffic is basically data traffic and SMS isn’t hiding in some unused space.

That being said it is still so close to free that it doesn’t really matter. Sending 140 bytes of low-priority data is a rounding error.