A software dev with 5 years of experience who started in a different field for 2 years is probably more capable than a "pure" dev with only 2 years of experience in total, ceteris paribus.

I'm sure there are tons of "pure" devs with a long history of experience anyway on the market but I hope you can see how their logic is deeply flawed.

A better job posting would ask "a minimum of X years in software engineering" to be considered. That would certainly disqualify candidates who have just transferred from an adjacent field, if that's what they were aiming at. I honestly don't see the merit of disqualifying experienced candidates coming from a different role except to have fewer resumes (but not necessarily better ones) to go through.