IC on Digikey is 5x more expensive than board containing same IC on Alibaba: How?

Digikey sources their parts from the manufacturers and adds a retail mark up.

The alibaba seller takes remnants from someone's production run (or possibly salvaged parts, or if you're unlucky counterfeit or reject parts) they don't pay full price.

Usually it's remnants, someone makes 10000 products and has 500 parts on the last 3500 part roll left over and sells that cheap, the module manufacturer gets these cheap, makes a bunch of modules and sells them.

I would say that they are probably counterfeit,this is a well known problem when buying from china, you cant be sure that they are original unless you are buying from a trusted chinese supplier. Most of the time they work alright if you are not pushing them to the spec limits,here are some links that can help you through this problem of chinese counterfeit IC and how to identify if what you are buying is fake