[Documentation] Custom selector engines before page is created

The documentation (both in code and https://playwright.dev/dotnet/docs/extensibility) states:
Selectors must be registered before creating the page.

However the code below works, in a TestClass inheriting from PageTest.
Isn't the page already created when entering this method, given that actions are performed on it?

[TestInitialize]
public async Task InitializeTest()
{
    await RegisterCustomSelectors(); // register here
    await Page.GotoAsync("/");
    await Page.Locator("myengine=test").FillAsync("hi"); // this works!
}

Start with the custom selector registration guidance in the code and the .NET extensibility documentation. Trace the PageTest lifecycle around TestInitialize, RegisterCustomSelectors, and Page to compare the stated requirement with the example. Done means both documentation locations accurately explain when registration is valid and why the shown test works.