Step counter doesn't count when I mow my lawn?

You are wrong. Look at the product page: https://www.garmin.com/en-US/p/775421

Says it counts steps. No asterisk. No footnote. No conditions.

See for example their Support document My Garmin Watch is Not Accurately Counting Steps

So why is this information not fully disclosed on the product page? There isn't even a footnote.

You are now telling me that Garmin are fully aware that there are many common situations where this device cannot accurately count steps and yet they still advertise counting steps as one of it's features. And I stress again: that advertised claim is without any footnotes or conditions.

Moreover, fitness goals are a key feature of this device. The manual says, goals can be, "...daily steps goal, daily floors climbed goal, and weekly intensity minutes goal." But again, you are telling me Garmin are well aware that at least one of these three goals will not work in many common situations because the step count is known to be inaccurate.

Look - your profile says you're not a Garmin employee so talking further to you seems like a waste of time. I just hope someone from Garmin sees this, realises what a major screw up this is and does something to fix it.

Funnily enough, that is exactly how I 'solved' the issue. Not really solved, but recording a 'walk' using GPS seems to also make the pedometer count (or at least estimate) steps as I mow. Although I doubt it is very accurate and clearly using GPS consumes more battery that really shouldn't be necessary. So yeah, sort of works but hardly a true solution.

It would be possible that if a device knew your stride, tracked your distance against something lie Google maps it would be able to assign a step value. 

If you can fit your watch on your ankle, you either need to quit skipping leg day or not work your forearms so much. 

So I have often wondered why there hasn't been implementation of a manual way to calculate steps over a specific distance to increase the accuracy in activities and I feel like this would be a good thing for mowing the lawn.  For example, I hunt which is much like hiking, but because I'm often doing my activity in early morning while its dark I know my pace count.  Before I had a watch like my garmin instinct 2 solar I would get to my trail head and I would start walking, knowing my average pace count per 100 meters over several hikes, it would tell me when to turn into the woods.  I would know the direction of my bearing and then I had another pace count in mind to get to my first marker which generally was a game camera.  Then knowing my pace count in a particular direction to my tree stand.

Point is I'm surprised for walking with a stroller or mowing the lawn there isn't a way to create a custom activity where it asked you to input your pace count in your preferred measure and before the activity was highly accurate it would need 5 manual pace counts recorded in that style or walking/ hiking.  Anyone at Garmin ever in the military? This is land nav 101 and it could be implemented pretty easy in your watches.

Same problem at work. Nothing is counted all day because I am pushing, pulling, and carrying objects while walking. I bought a small pedometer to drop in my pocket, but there is no way to add those steps in the Garmin app like there is Fitbit. It's frustrating.