A lot of comments here positioning fish vs humans. There is no doubt everyone has something of a personality.
However this study was performed on genetically and developmentally identical siblings, which is impossible
to do in humans (at least ethically) and the interesting part is there was still individual variance, present from the time of birth, that increased over time.
To emphasise this point: in almost all other animals, a profound level of motor specialisation is present at-birth. Most animals, are eg., born walking.
Whereas in humans, we are born needing to specialise almost everything during our lifetime -- which has incredibly high costs (ie., many years of being fed without being able to hunt) -- but high rewards: we learn how to conceptualise the world in novel ways, create tools, etc. both largely a product of our ability to specialise our sensory-motor system in response to the actual environment we're in.
There is no reason to suppose that we are analogous to fish in our personalities, but not in everything else -- indeed, there's obvious reasons to suppose the opposite.
Human babies are born significantly earlier in their development due to birth canal width VS head size. Without this limitation, they'd be born after 2 years or even more.
Things we take for granted like swallowing, or sneezing involve a great deal of complex and precise motor coordination that babies can do right from the get go.
Gene expression is dependent on incredible amount of variables. It'd be a cosmic level of coincidence to have two clones express the same way. That never happens even in single-cell organisms, much less something as complex as fish or human.
However this study was performed on genetically and developmentally identical siblings, which is impossible to do in humans (at least ethically) and the interesting part is there was still individual variance, present from the time of birth, that increased over time.