Yes, for the 13" the video is actually being displayed at 1440p when fullscreen, meaning 1080p is being upscaled to 1440p.
I still tend to prefer watching in 4K (2160p) over 1440p on a 13", because although you can't really take advantage of the extra pixels, you will typically see improved quality from a higher bitrate. But may not really be noticeable like going from 1080p to 1440p (or 1080p to 4K if 1440p is not an option).