also this: apple systems are not hard realtime. page fault occurs - nothing will stop audio glitch. and this can happen regardless of what language you use C or swift.
you can measure how many glitches say, per hour you are actually getting - inputProc/renderProc has timestamp parameter and if the previous timeStamp.sampleCount + numSamples doesn't match the current timeStamp.sampleCount - that's a glitch. do the two barebones implementation of, say, square wave generator, one in C, another in swift and compare the actual results on a couple of platforms. i wouldn't be surprised if you get no glitches in both implementations. or if you get a few glitches per hour - again in both implementations.
take Apple advice of 6-10 years ago with a grain of salt, especially given they stopped non recommending swift for real time audio 5 years already. obviously i am not saying it's good to use swift containers, or async dispatch or mutexes, etc... just read memory / write memory and math.