Yep this so much. It is not that code is brittle but problem space is hard. What seems like an obvious fix may break other vendors that depend on same behavior etc.
Having said all of that. Modesty is a thing and many engineers wouldn't word things the way Manish worded in aforementioned blog.
Deep familiarity with a complex code base is a thing and is not easily replicated. I have seen this first hand. In my day job - I work on a very popular but complex open source project and in my team there is a engineer who has been there since beginning and I am like second person after him.
I have noticed that when we get certain class of bugs from customers(and customers want fix NOW) that spans 2-3 layers of the project, only me and him can pick those bugs. Some decisions are there in the code that is not easy to figure out(like why it was done this way) and I have also seen engineers who are founders of this project - point design issues that I could not have thought (and I am doing this for 5 years!).
Can we hire someone new to provide design reviews that spans network, storage, kernel? May be - but engineers who created this thing and have been doing this for long time, have got really good at it. To me - some of them seem god like. I know they will probably suck same as me, if they had to start developing a new iOS app or write new game for Nintendo Switch.
Having said all of that. Modesty is a thing and many engineers wouldn't word things the way Manish worded in aforementioned blog.