From what I can see the driver is reporting that several things are not implemented.
Read of Altitude
From a look at the LXP driver source this is an early driver which only implements version 1 of the ASCOM protocol, the first three of these aren’t in version 1.
There’s no indication that the mount can only slew in Alt/Az, more that it can only slew to Ra/Dec coordinates.
The driver implements read of right ascension and declination, slews to RaDec positions, syncs to RaDec positions and PulseGuide but not much else.
That’s enough for a simple program such as CdC to be able to read the mount position and do slews but SGP needs more than this. It doesn’t mean that the driver is not ASCOM compliant, more that it’s not SGP compliant. SGP needs more than ASCOM compliance.
It’s a shame but a lot of mount manufacturers don’t seem to do anything to support external control of their mounts. A lot simply implement the LX200 protocol - more or less - and hope that someone else will provide a driver.
It’s possible that a different driver that supports the LX200 protocol will work, but I don’t know which.