No they didn't? They released the phone before official Android 15 roll-out, the article you link to states various plausible reasons to that launch was delayed. Also Google's pledge is years of software updates, not that it would be a number of major Android OS.
Pixel's official launch date for Android 15 is October 15th.
And do note that iOS requires that e.g. safari updates are via system updates.
In Android more and more of the system is being updated via the Play Store instead, this includes stuff like the android kernel (Google promised at least 6 years of updates for
Project Treble enabled devices ~2017 / devices that launched with Android 9 (Pie) all get kernel upgrades) and includes browser updates for at least the last decade.
With Android 10 this became Project Mainline, and every major android OS has moved more components to be modular, list here:
source.android.com
And as more and more components over the years have been added to that, it means all users to get new features that e.g. allows instant WiFi feature:
EDIT:
And Android 15 should probably add NFC to the mainline stack.
Should also add Thread (smart home communication protocol, Pixel 9 has the radio built in), and UWB (ultra wide-band, e.g. should help with finding lost earbuds and stuff).