> so can any party you trust
Yes, but the difference is that other parties I trust are subject to external verification and audit. If my bank steals my money, it will soon be discovered, either by me the next time I get my statement and it doesn't match my records, or by the bank auditors the next time they audit the books and discover that they don't balance. By way of contrast, if Apple sneaks a back door into a MacOS update it could easily go unnoticed for years.
For most values of “me” as a customer, it’s in Apple’s interest to behave as they advertise. They can stray due to government or other action, but so can any party you trust.