Is the Transit VPC the legacy equivalent to Transit Gateway?

In a Transit VPC architecture, you essentially need to create a mesh network to allow all spokes to communicate with each other, with a total of n(n-1)/2 peerings, given that VPCs are non-transitive by design. The Transit Gateway provides a central hub/router that allows spokes to communicate with each other without having a direct peering, and allow communication via the Transit Gateway. It significantly simplifies the network!