These horses are living wild (feral) in a protected natural area with nice beaches to sunbathe on and no natural predators.

Yes, for the most part, these horses are living exceptionally good horsey lives. Far more fortunate than most horses.

(Though there definitely are some downsides. There’s nobody to take care of them if they get injured or sick, nobody to give them dewormer and rid them of parasites. On average, they’re probably not living quite as long as a well cared-for domestic horse. But maybe the freedom to roam and reproduce as they choose makes up for that, as the vast majority of domestic horses are not allowed to do either of those on their own volition.)