I'm not well versed with what counts as isekai and what doesn't but its definitely not a dream or premonition. The MC did travel back in time and transform into his old self in all but mind and cock and I do think these ~40 years of additional life experience are kind of necessary to justify the way he's acting during the story or his twisted memories(!). After all, if it where just an dream or premonition, there wouldn't really be a reason for him not to remember all that stuff.
To be honest though, I didn't put too much thought into this whole time-traveling thing. It kind of just happens. In my mind there isn't any goddess or anything like that but as it won't every be explained anyway, everyone can feel free to think about a how or why for them self.