[mock-possum]

I think it's being used in the sense of 'the industry' discouraging dissent about the universal applicability of JS application frameworks and SPAs, by trying to convince developers that their experiences with SSR and MPAs are not real, or not relevant. That yes, plain old html and css and vanilla javascript feel like they USED to be good enough, but they're not good enough anymore - for anything. That's the gaslighting bit - when you're instructed to doubt or de-value your own experiences and feelings. It's done on purpose, with intent to control.

Which, along with the author's "you were sold something" angle - seems perfectly appropriate. He's saying you were lied to on purpose, you were told to doubt your own experience. That's gaslighting in this context.