Today, we are announcing AWS Direct Connect support for AWS Transit Gateway. With this feature, customers can connect thousands of Amazon Virtual Private Clouds (Amazon VPCs) in multiple AWS Regions to their on-premises networks using 1/2/5/10 Gbps AWS Direct Connect connections. Until today, AWS Transit Gateway only supported AWS Site-to-Site VPN and Amazon VPC attachments. You can now use Direct Connect gateway as an attachment with your Transit Gateways.
from What’s New https://aws.amazon.com/about-aws/whats-new/2019/04/announcing-aws-direct-connect-support-for-aws-transit-gateway/