I think the rut being referred to is an inability (rooted in complacency and habit, not actual inability) to learn, change, or adapt. Nothing to do with specific technologies, or even new vs old technologies. Just a far too strong inclination to stay in one’s comfort zone, and for that comfort zone to never expand.
I’m pushing 50 and know many older engineers. An occupational hazard is getting complacent and ceasing to evolve and learn as an ongoing part of one’s career. This is a hazard with life in general as people get older, but if we’re not wary of it, it can have professional consequences too.
As far as what does it look like- they go to a new team or employer, which naturally has different tools and practices and technical domains, and they expend titanic amounts of effort to continue working just like they used to in the new surroundings, rather than simply making an effort to adapt and learn new skills.
I’m not saying this is a common problem when hiring older engineers, just something we all have to grapple with as we age. I think that’s all that was meant by a rut. Resistance to all change, regardless of whether that change is good or bad.