Technically, it’s FedLand, not Texas. It’s surrounded by Texas, but it is Federal land and Fed laws apply.
Just like any military base.
Has Texas surrendered its rights to the Johnson Space Center? Otherwise the mere fact that the Feds own it does not make it a Federal enclave. Because Texas was a sovereign nation before joining the union, the Federal Government never "owned" Texas land.
I know of Military Bases where the State of Texas has surrendered its sovereign authority, so a DWI there can be a Federal crime. I wouldn't be surprised if the Feds demanded such a surrender in exchange for the billions they have spent at Johnson Space Center.