I thought about this, so I wasn't seeing the sponsored tweets ... but I don't see why they couldn't have just modified the feed they provided to Tweetbot, Twitterific et al to include these ads.
I agree that Elon doesn't care, but he probably should care that heavy users like me can be so quickly and easily lost. And in addition to being more bad PR, 3rd party developers will interpret this as a clear signal not to invest much in integrating with Twitter.
With a third party client Twitter can’t (easily?) measure if an ad was actually seen by the end user (impression) or if it was just served by the API but never rendered on the users screen. A lot of ad measurement follows from there.
I agree that Elon doesn't care, but he probably should care that heavy users like me can be so quickly and easily lost. And in addition to being more bad PR, 3rd party developers will interpret this as a clear signal not to invest much in integrating with Twitter.