In short, yes, you are correct.
A transit gateway is a "normal" AWS account and VPC. Inside of it is a transit gateway that allows it to connect to other AWS accounts or VPCs.
If you place the VMX into the transit gateway VPC then you can add static routes pointing to it (for the remote Meraki sites) and on the VMX pointing to the VPCs.
I haven't done one of these yet myself, but I have had a number of people ask me about them.