Your link is for the current OS only, but I found details about iOS27 in the newsroom.
There are a few subtle differences that are probably crucial. Both iOS26 and 27 use Private Cloud Compute. For 26 they specify that it runs on Apple silicon servers (not necessarily owned by Apple) and that no personal data is shared with Apple. For SiriAI in iOS27 it says that data is processed on servers using Private Cloud Compute (without specifically mentioning Apple silicon) and state no data is shared with Apple or others. Which would imply it runs on Google's servers who are the 'others' with potential access.
For me that is too much ambiguity, but I look forward to people more capable than me investigating and testing these claims thoroughly