That page doesn’t mention using <object> as an alternative to <iframe>
https://www.w3.org/TR/html401/struct/objects.html is the standard that mentions it - note that with HTML 4 iframes required a transitional doctype while the object equivalent was allowed with the strict doctype - indicating that object was the preferred option for future use.
The two are equivalentfor embedding web pages in all browsers more recent than IE6 (which forced a border around the object that couldn’t be turned off).