It's a very long stretch from "generate hydrogen" to "powering existing fossil fuel plants".

The most unlikely part is not even creating renewable fuels (that is a stretch already), but the idea that those fuels are going to be compatible with existing plants and infrastructure. It's not impossible, but it would probably be the least economical way to go about it. I recommend reading some industrydecarbonization.com articles for going a bit more in-depth about the why.