I assume this works via emailing the carriers (e.g. number@vtext.com),. If so, why not just release the source (or is that somewhere else?) over running your own service?
It looks like that's how they're doing it since they have the caveat of "may receive messages from txt@textbelt.com"
As for the open source comment -- they're probably running a carrier lookup on the numbers via another API on the back-end, so that dependency on another service doesn't make it as straightforward as just open sourcing it.
I would be interested in knowing what API they are using for the carrier look-ups.