The Texas grid does have interconnects with the other US grids and Mexico. However the capacity on those is insufficient for current demand.
In power grids an interconnector is used to move large amounts of power between grids and if it can't do that then is not an interconnector.
The Texas grid is and independent grid control by ERCOT exactly for this reason.
It was always designed to be independent by the Republican government that has controlled Texas for the last 20 years.