Same way Siri understands the english “Los Angeles” even though the G sound is completely different from Spanish.
The english IPA for Bach is straightforward.
Not only Siri — the whole iOS. You can’t type a sentence switching languages in the middle, without changing the keyboard language all the time, if you have autocorrect enabled. It will change what you type into utter gibberish, even though without the “correcting” what you type is perfectly correct. This system is quite visibly designed by people who speak only one language and don’t understand that people may want to use multiple languages at the same time. The keyboard should support a mix of languages, instead of making a XOR between languages, because otherwise when it starts, it’s almost always in the wrong mode, and if it isn’t, it will almost certainly be wrong by the end of what I write.
The audio clip the person posted was for a true German pronunciation, which happened to be very different than how 99% of English-speakers would say it.