Single page application support can be toggled in liferay-portlet.xml, not portlet.xml
This sentence should be updated in the article below:
If you prefer, you can set this property to false in your portlet.xml instead by adding the following property to the <portlet> section:
https://help.liferay.com/hc/en-us/articles/360030752391-Disabling-SPA#disabling-spa-for-a-portlet