[ZephyrBlu]

I read it like this:

- "We extend SAM to video", because is was previously only for images and it's capabilities are being extended to videos

- "by considering images as a video with a single frame", explaining how they support and build upon the previous image functionality

The main assumptions here are that images -> videos is a level up as opposed to being a different thing entirely, and the previous level is always supported.

"retrofit" implies that the ability to handle images was bolted on afterwards. "extend to video" implies this is a natural continuation of the image functionality, so the next part of the sentence is explaining why there is a natural continuation.