That device at the two tuner inputs of the 622 is called a "separator" and it's completely different from a combiner/splitter. Make sure the diplexers you're using are power passing through the sat. connections as you'll need to be able to power the LNBs from the 622. (Not all diplexors are the same!)
The installer was implying that there will be some signal loss through the diplexers (it's inevitable, on both the sat. amd OTA signals) that could result in some deterioriation of the picture quality. It's not likely to be a problem for the sat. signals, but could be an issue with the OTA signals. In general it's best to run separate cables for OTA and sat., but if you can't do that then take your chances with the diplexers. There are ways to boost the OTA signal in the combined scheme, but that requires a preamp that will work from the same power that the LNB uses and also a diplexer at the antenna end that passes power to both ports. If your OTA signal is less than optimal you might want to consider that option.
Good luck with it, but you shouldn't have any problems.