The piece did not say "third party". I'm assuming these clients are the official Twitter mobile apps.

Probably their mobile clients. After I read the article perhaps Twitter felt that having control of the clients reduces variability in stress to their system. Like bad actors accessing the firehose suboptimally.

The two are not the same. It sounds like they are becoming API driven to support multiple interfaces and to put clear boundaries between systems.

Sounds like html. Or qt. Not 140 characters.

Yeah, somebody hasn't gotten the memo it seems.