Any Turing Complete language would "fit the bill", obviously.

What's wrong with a company not just kicking the tires on another language, but using it on a big project? That's how you design better languages of your own.

"Go was great, except for X and Y" leads to improvements on your own stuff.

"I like X, but i fail to see why Y and Z don't fit the bill as well."

Try repeating your point for any order of X, Y and Z, and you might realize it isn't really a valid reason for them to have used Swift or C++ instead.

Apple owns C++ and Swift compilers.

Your original argument was that C++ and Swift fit the bill as well. Now, as far as I can tell, you seem to be saying that C++ and Swift are better choices because Apple owns compilers for them. That is a different argument, which is in direct conflict with the one I originally responded to. My response was based on the assumption that you meant what you wrote.

Anyway, onto your current stance. That Apple don't use Swift or C++ for their spider seems to indicate that the benefit of using Go outweighs the benefit of using a language they own the compiler for. Go has a BSD-style license, so as far as Apple implementing their own changes and doing whatever they want with the source code and their own binaries at will goes, they might as well have owned it. Go is also seems like a no-brainer choice for a web spider since it has concurrency modeled in such a straight forward and cruft free way.