Let's be serious. No one reads twitter over sms anymore. This isn't at all the reason twitter continues to limit to 140 characters.
(Before some smartass replies "I do read over sms", well even without sms merging, twitter could still split a tweet into multiple sms. 99.9999% of their userbase wouldn't care.)
That's not relevant to the OPs point. The point is that even SMS clients were capable of merging multiple messages together, yet Twitter has still failed to do so. So whether or not they were SMS doesn't matter.
SMS has for ages, thanks to Nokia, had the capability of merging multiple messages into a single large one.
Twitter could just make use of this and save themselves a whole lot of grief.