We are talking about computing in 1985. A 25 MHz workstation would be at least 10,000% slower than a modern computer, with about 1,000× less memory available. Still, this isn't a huge difference in context, but talking about adding 4 to 16 bytes of overhead on every data structure, or every object, in a worst case scenario could result in twice as much memory usage.

Steve Jobs wasn't an engineer technically, so he wouldn't know the specifics here, but at the time object-oriented programming had a stigma associated with it because programmers did know of these drawbacks. Using Objective-C was obviously a fundamental choice by the software engineers.

Writing C in an object oriented style is what a lot of programmers did then. But there's a difference between associating data with methods and supporting polymorphism. Hence the talk of virtual function tables.