why does an m1 Mac think that a side loaded iOs app is jailbroken?
The App is doing this check, not your computer. Apps use a variety of techniques, usually involving checking for the presence or lack of specific files that indicate the possibility of a jailbroken iPhone.
This app has implemented their check poorly, causing it to trigger on your Mac. You should contact the developers of the application and tell them to remove the jailbreaking checks.
Note: Of course, if the developer did not list it on the App Store they may not be receptive to making changes to accommodate running it on the Mac.