I'm baffled how you think a senior can just sense the codebase without reading and effectively explain it even to a junior, who as you just described can barely understand the code. I think you are describing a topcoder who is working with a small, well written codebase and is familiar with the domain and stack, most of which in my experience is quite rare.
You’re painting it too binary. Once you’ve seen a few dozen codebases you get pretty good at recognizing the patterns. A senior engineer could/should absolutely be able to familiarize themselves with the general structure of a new codebase in a matter of minutes/hours to the point that they could reasonably start directing a more junior engineer within it.
It’s not “go to this file and do X, Y, and Z starting at line 105”, but more like “Ok so it’s likely that the source of this data is coming from a feed, track down the feed consumer by grepping for [upstreamservice] in the feeds directory, and from there we can get a look at what we’re doing with the incoming feed data”
That's different than not reading the code. Technically, that's applying tool assisted reading techniques to quickly determine what subset of code to read.
I still find seniors are terrible in that they assume they know what code is going to do without having read it, or they refuse to learn the domain first.
I've seen this happen but without that "without reading" part. I've had a coworker, who about a 1-2 weeks after starting working in erlang (without prior exposure) on our big codebase was already able to contribute some fixes and soon new functionality and he was not even considered as senior dev when hired. Most of my coworkers are experienced and also able to work on new codebase pretty soon if required. They are not that rare.
Every codebase I've seen has a ton of brand new WTFs in it. If you've been doing it for a decade, you've seen between 1 and 8 codebases. A mix of "yeah yeah everyone does that" and weirdness bespoke to that culture. The "yeah yeah everyone calls 'new' to get a new object" isn't what needs fixing.
Seeing 8 codebases in a decade is a bad sign. I've significantfully contributed to maybe 30, contributed to at least 100 others, and read large chunks of 100s on top.
