A few thoughts, in no particular order.
Correct that "co-existence" has to do with Bluetooth and wifi co-existing in the 2.4GHz spectrum, however I would suspect a windows driver issue or something similar that's causing the machine to run 1x1 (if it even is). The existence of Bluetooth (especially newer devices) is not very impactful anymore and there are good interference avoidance mechanisms. Plus it's only going to have an impact (even if only minor) in the 2.4GHz band, not the 5GHz band.
What is the machine capable of, 2x2 or 3x3? What gets reported (maybe the same output) running that command when associated to other wireless networks or APs? Every time I've ever run that command I usually see "Unknown". for coexistence support
Also check what throughput results you get if you connect to the AP from the machine directly, using the local status page by pointing a browser to ap.meraki.com and running a speed test, and see if it's indicative of how many spatial streams the client device is capable of.
Also check what you see on the Network Wide > Clients page for that client, and open the "details" link under device capabilities and see if more than one SS is detected. Or on the Clients page, turn on the "capabilities" and channel width columns. Also look at the MR53's AP page where it lists current connected clients and you'll see what channel it's on. Turn on band steering and see if you can get the client to connect on a 5GHz channel, and if/when it is, run your netsh command again, and if it still shows that same output, I'd disregard it, not an issue.
Also, I'd assume the MR53 is running in full PoE power mode (802.3at)? If it's running on legacy 802.3af power, it will disable a spatial stream plus the 3rd dedicated scanning radio in order to run in low power mode.
