No it indeed isn't as you can't really configure anything about it.
Same goes for the switches where it is documented:
https://documentation.meraki.com/MS/Layer_3_Switching/MS_Warm_Spare_(VRRP)_Overview
"At this time Cisco Meraki MS Series switches can only be configured in pairs from the same family when using VRRP/Warm Spare. Integration with other vendors or platforms is currently not supported.
Ex. An MS250 can only be paired with another MS250."