I can only conclude that any device you test on that fails iSCA isn't supported. Pretty simple, regardless of platform, so while iPad is your focus, I suspect there are unsupported iPhone & touch models as well. Have to ask, which iPad are you testing on, exactly?
Sorry, but I am unable to locate a reliable citation for which devices are supported and what makes them special in this regard.
Note too that not all iPads are created equal, so... I'd wonder if the info isn't public because someone in charge doesn't want to go on record that strapping an iPad to your arm may not be the most flattering product endorsement.
And before someone suggests they see step count apps in the store that claim compatibility, I suspect that claim is more about a given iOS version and while they may run on devices that fail iSCA, in those cases the app provides a crippled feature set.
I wouldn't look for an alternative (unless this is about re-purposing step count as a means to another goal, perhaps), but hey, I'm not anxious to be seen power walking with a tablet on each arm just so I don't wander in circles 😉