Also, in my experience, stagnation can run in the other direction, temporally. I encounter many young people who won't even learn about, much less consider, using a tool like 'make', simply based on the assumption that newer seemingly similar tools must be better. Sometimes the old ways really are superior for the task at hand.
Most of the successful tech people I know that are 50+ never overstayed their welcome at companies where they stopped learning, or they were always being recruited by other firms and moved around. Recruiters generally aren't knocking on doors of someone who has been at the same company for 20 years, and that's for a variety of reasons.
Not the way you're describing it. However, there's a pretty common correlation between age and relevance of skillset, which is a direct consequence of stagnation over many years.
The point is that someone saying "old == stagnant" may actually be thinking of it the way I'm describing, which has more validity.
Younger ones can have all the latest Javascript library experience they want but they still don’t have enough obvious experience for engineering mastery that’s vital to being a highly-productive independent contributor or tech lead.
Read it again. It's referring to older stagnant people, not all older people. The longer someone has been stagnant, the more out of date their skills become.
Younger ones have the benefit that their skills are still likely to be relevant at a larger subset of the employer pool.