a7S III was overdue over a year ago, but they felt the need to voice their plans only now. What changed?
a7S III was ready for production (not just concept-design stage) about last year and awaiting competition for price adjustment. "We are thinking about making one" is probably just to disprove this suspicion.
Competition did something (or is about to do something) that required redesign of a7S III and it's no longer about waiting game, which they lost.
My thoughts are: they have a superior a7S III version designed for scenario like this, but production is not ready. It takes about 5 to 9 months to set up a production on a large scale if it's for key elements (sensor, CPU etc.). There is no way they issue a statement about the camera that is nowhere near this stage. So I'd expect it no later than within a year, may be even announcement at NAB.
I wish Canon would come up with a Pro RF-mount body by then so we have more FF options to choose from by the time Sony shows a7S successor.