Interesting article. Thanks for posting.
I have thought that Indigo would actually codeshare across the Atlantic, but I always thought they would need A330s to do it. Being able to stop in Iceland fixes that problem. A frontier A321neo lands in Iceland and the passengers hop on a Wizz 321 to anywhere in Europe.
By owning WOW, they can freely do that.
The Frontier/Volaris codeshare is likely just a trial or experiment for a possible trans Atlantic partnership.
Someone who is going to a future Roadshow should ask Bruce York about the scope provisions involved with such a setup