...yes. It's pretty common to talk in terms of worst case...
The unicode extensions for SMS only allow 70 UCS-2 code points. So from the perspective of ASCII, SMS can too big to fit into a tweet, and from the perspective of Unicode a tweet can be too big to fit into SMS.
Tweets to me look like a product manager liked the constrained aspect of SMS, but didn't dig hard into the details.
At Remind we operate one of the largest SMS delivery systems out there. We also have a 140 character limit, reserving 20 characters for the handle/class name.
In aggregate, having the limit is a cost decision. We absolutely can and do send messages longer than 140 characters. But we make a best effort to limit it to a single SMS segment. If some are over, oh well.
When Twitter first came out, in the US it was common to pay for receiving SMS not just sending. Multipart messages weren't always handled perfectly before modern smart phones, often they would be delayed or be received in the wrong order.
interestingly Europe was all over SMS messaging in the 00s, but it didn't really become popular in the US until this decade, when carriers starting bundling unlimited SMS in plans.
