It's interesting because the example he makes (a person that has worked 5 years on the same things, vs one that has had more varied tasks) makes me think the op

It's interesting because the example he makes (a person that has worked 5 years on the same things, vs one that has had more varied tasks) makes me think the opposite of what he claims: the person working the same job for five years gained five years of experience in that job; the other one, has a one year experience in each of the various things he did. Of course there's a good balance between repetition and variation, but beware of those who have written terrible junior code in a lot of different technologies.