I think this "Apple's new linker is in the same league as mine. Great job!" response is full of tact and grace, considering Apple just undercut his business.
I dont think he was willing to sell to Apple for what Apple would consider as reasonable price anyway.
Large companies (Apple, Amazon, and the like) would likely buy the company rather than license their product (see Dark Sky (now WeatherKit), Kiva (now Amazon Robotics - https://www.fromscratchradio.org/show/mick-mountz 23:22 )) so that they can't lose control over something that they want when management for one of there tools changes.
And so why use mold (license problems and they wouldn't want to impose the AGPL on all their developers), or be a reseller of sold ($8/month/user - how many developers are in the Apple developer program) - it comes down to "it is better for us to develop our own and release it so that there is certainty about the license for any developers who use it."
It would even be further beyond the pale to say "here is a linker for 3rd party developers to use... oh, and its AGPL, sorry 'bout that."
So to the question from the GP:
> Why didn't Apple just use mold?
Because they won't tell 3rd party developers to use AGPl software or require a 3rd party developer to use a different 3rd party developer's licensed software that may change.
If Apple is going to release something, they're going to develop it and release it themselves with as few other encumbrances as possible.
I remember an SFC blog post[2] expressing distaste for such scare-tactics applications of copyleft on the part of MySQL AB in the early 2000s. (As opposed to the initial goal of copyleft, that is using a legal hack to eventually force most software to become free/libre.) This case is maybe less unpleasant, but only marginally so, I think.
[1] https://gist.github.com/lleyton/9c0b75d065f37333ea9851b6cad1..., discussed today at https://news.ycombinator.com/item?id=36225016
[2] https://sfconservancy.org/blog/2020/jan/06/copyleft-equality...
Now, thinking about that case, I guess that Apple could make it easy for people to depend on AGPL software that they download on their own, and that Apple only provides some sort of index or crawlers for it. No need for Apple to have AGPL software on their end.