Thanks for the welcome ?
Basically, I want to compare the "created" timestamp of the new page with the "created" timestamp of another page.
I have since found out that I can do it by calling $pages->get() with the id of the newly created page:
$testPage = $pages->add('basic-page', '/', [
'title' => 'This is the page title',
'body' => 'Lorem ipsum'
]);
$newPage = $pages->get($testPage->id);
var_dump($newPage->created); // result: 1618991002
I don't really understand why I can't get at it directly though. If $testPage has an id, I would think it has been saved to the database, and all fields would be populated. But hey, whatever works ?