Don, yes it certainly does have something to do with TX not being on an out of state grid. That was imposed way back when NYC had the massive blackouts back in the 60's I think from under frequency trips.
TXU claims they did not expect the heat wave, so they did not bring up generating capacity to meet the demand and got caught short. Not sure how they could have missed the heat wave, it was forecasted a week ahead of time
When I worked for a Oklahoma utility, in 1980 something, I worked in a AC-DC-AC interconnect with TXU. Basically a two back-to-back 69 KV substations in the same yard with rectifiers and inverters.. So the state of Texas does have some interconnect to the outside world, they just are not synced up with any other state or grid other than their own.