> could something of this magnitude be powered by renewables only?

Perhaps.

For context see https://masdar.ae/en/news/newsroom/uae-president-witnesses-l... which is a bit further south than the bulk of Texas and has not yet been built; 5.2GW of panels, 19GWh of storage. I have seen suggestions on Linkedin that it will be insufficient to cover a portion of days over the winter, meaning backup power is required.