There's a difference between "feeling unproductive" and actually being unproductive. The person you're responding to said the former, not the latter. And in my experience it's nearly universal that when you ask excellent and very senior engineers "how long did it take you to feel ramped up?" they answer "I still don't!" despite having been there for months/years.

Had few jobs in low level system codebases in the millions of locs. I've had 9 months probation periods.

Nine months is insulting unless you guarantee me a VERY generous severance if things don't work out.

There’s a reason people refer to it as “ramping up”, as you become familiar with the environment and code base you become more and more productive until more or less plateauing a while later. It’s not really useful to categorise it as binary productive or not productive.

So you are telling me you never qualified at a workplace with longer than a month probation period?

Do you see how that putting-words-in-others-mouths looks? It's rude and does not contribute too any kind of productive discussion.

Depends entirely on the system you are working with.