That’s odd. Right now I’m at the memory warning level (not the hard stop where you are), and I get the same message trying to add a GE Z-Wave Switch by brand as doing the Ikea Motion. Not sure what to make of that, unless you already have the stock driver for the Ikea Motion running and the hub knows it can handle another device on that driver.
My understanding is that those memory warnings are RAM, not storage, so what matters is the running drivers and not so much any that aren’t in use by any devices. Below is a quick summary of what I’ve found on my own hub regarding the memory issues. If you have 20+ drivers in use then if you watch logcat on all drivers I think you’ll see a driver restart every minute as the hub tries to stay under the memory cap for drivers.