Acquiring incremental engineering skills is great and we should all do it. But let's admit that it's just that - incremental. You go from being a good engineer to slightly better engineer- something you already are.
When you pivot to management or something else that's more different to you - you are taking on a bigger challenge and there's no guarantee of success. In order to succeed you need to hone a whole different set of skills - and if you manage to do that, you will certainly have grown in a real way.
I totally get the sentiment that going into management isn't the only way to advance a career and I agree. I am just saying that those who take that path indeed open themselves up to a challenge and evolution.