It is an idiomatic expression which often defy logic or formal usage.

You already know, that it is implying an emphatic positive assent.

"The sun is boiling today."

 

"Golly, and how it is boiling today."

It reminds me of the ironic, "You think?" and the (way outdated) country expression "Boy howdy!" both of which express enthusiastic agreement.

It is an idiomatic expression which often defy logic or formal usage.

You already know, that it is implying an emphatic positive assent.

"The sun is boiling today."

"Golly, and how it is boiling today."

It reminds me of the ironic, "You think?" and the (way outdated) country expression "Boy howdy!" both of which express enthusiastic agreement.