TX does have its own grid, but it is able to import power from the eastern grid (not the western that CA is on, though). Moreover, in the middle of the winter, what was equally important was that the natgas pipeline capacity was shared with other states (and Mexico), and that's what had the biggest failure, because many others were using record amounts of natgas at the same time TX needed it, in part to fuel electrical plants. Unlike, for example, coal, where you have a big mound of it next to the plant, and sure it's brought in by train but it's not expected to show up just-in-time.
All of which is to say, being on a larger grid only helps if everyone else isn't peaking at the same time you are.
Also, they didn't (and still don't?) pay to winterize plants, so if it gets mildly cold by northern US standards (which happens about once a decade) a bunch of their plants (and unwinterized windmills) shut off.
All of which is to say, being on a larger grid only helps if everyone else isn't peaking at the same time you are.