Specific providers make those guarantees, to go beyond just selling green credits they already have lying around. I think in Texas Bulb should qualify in this case.
I'm not really seeing anything on their site that says every new customer's kW-h usage translates directly to new kW-h sources being built. They do say:
"For every unit of electricity you use, we put a unit on the grid produced by a renewable source like solar or wind. "
But that's only talking about generation from their existing resources - not the building of new resources. In fact they go on to say that they're only purchasing energy from renewable sources to one-to-one match their customers use. Bulb is just a reseller like any other.
They also seem to go beyond the bare minimum and have more information.
I thought the Bulb site was implying they bought bundled RECS, i.e. they paid local Texas renewable developers for both their energy output and the REC at the same time.
https://bulb.com/