[jfz]

The author probably meant that they don't believe that full-stack engineering is beneficial compared to hiring primarily specialists.

[cam-stitt]

Potentially. Which is why I believe that it is more of a click-bait title than a meaningful discussion point.

[pseudalopex]

It's a common idiom. Someone who says "I don't believe in psychics" means "people who call themselves psychics don't have the abilities that implies", not that nobody claims to be one.