That is not correct. You can set solo with this command. I've done it successfully in the past, and can find mentions online of others doing the same.
I'm admittedly rusty with Qlab OSC; most of my OSC stuff has been in other contexts, and I know that the X32 OSC has some formatting quirks that might require some experimentation or modification. But some variation of this (turning solo on channel 1) should work:
/-stat/solosw/01 ,i 1
Searching online I found at least one report of an X32-Edit app bug causing the app to not reflect use of this command correctly, even when the actual console IS reflecting it properly. So if you're monitoring your changes using the X32-Edit app that might be why you're not seeing a change. Look at the actual console, or monitor with other OSC commands like /-stat/solo.