From what I see out of a UTM "host only" network, it is creating its own local virtual network (as evidenced by the unique network bridge and vmenet devices for hosts using that virtual network as shown by the ifconfig command on the host). VMs running under UTM can communicate because that hypervisor understands that a 'host only' network should use that local virtual network that UTM created.
Fusion's host only network does something similar for its host-only networks. It creates a virtual network segment for its host only networks. The problem with what I think you're trying to do is that Fusion creates a separate virtual network segment (from ifconfig, you see that there is a separate network bridge and associated adapters per VM for its host-only virtual networking). Your Fusion VM using its host only network can't talk to any UTM VM over its host only network because you have two independent network segments with no routing or bridging between the two.
IMO what you want to do might take some tinkering with bridging and/or VLAN configurations...