Meaning of "and everyone, but everyone, was calling"

Question

I recently took a year or so out to return to school. This is something I recommend for all sorts of reasons. One benefit is the new perspective you gain afterwards when you revisit a once-familiar world. I came back to consulting and found that most of my old clients and contacts had taken the plunge and migrated to Git (I follow suit in this edition). And everyone, but everyone, was calling their development methodology agile. However, of my first four new clients, three asked me to help evaluate hastily-built and inflexible codebases.

How does but everyone fit in?

Answer 1

It intensifies the first "everyone". If it simply said, "And everyone was calling their development methodology agile", one might assume the writer meant "a large majority of people" or "nearly everyone". By adding the "but everyone", the writer is say "effectively every single person".

Answer 2

It's an intensifying idiomatic parenthetical phrase which defies the usual kind of parts-of-speech analysis.

Answer 3

The sentence is essentially stating that everyone's (as in, each individual as a singular entity, not a whole) own views of their own work is the right one.

Here's a break-down:

And everyone,

All the people.

but everyone,

Everyone is now excluded, meaning it's down to the specific individual.

was calling their development methodology agile.

The subject.

In this context, it can be rewritten as:

Each person thought that their own specific development methodology was agile and disagreed with the methodologies of the others.