This is bogus. The Reddit user has been deleted and if you read the comments, you will see other users replying that the max resolution Eufy has supported has always been 1080p for RTSP. They have only supported 2k on the local SD cards and the Eufy app. This has been known for a long time. The app message is more recent, but still several months old. I have never seen anything but 1080p in the 6 months I have been using RTSP.
Here is a link from July last year detailing how to set up RTSP to a NAS. Notice that it specifies 1080p resolution at 15 frames per sec. in the setup.
RTSP Streaming with Eufy cameras and Motion](RTSP Streaming with Eufy cameras and Motion):