How to Search for an Exact Phrase in Outlook 365 Online Archive?
Solution 1:
The article you found explained how to find whole words, not phrases.
To search for a phrase, just type it with quotes in the Search box.
For example, type "this is my search"
in the Search box.
This will search all emails in the current email folder which contain the entire specific phrase.
To search in all email folders, click "All Mail Items" under the Search tab.
For more details with screenshots see the article How to search entire specific phrase in Outlook?
Solution 2:
I can confirm that searching for "any phrase in quotes" is broken in Outlook 365. Sometimes it works, sometimes it doesn't.
When it works: search a specific folder, or select the "Current Mailbox" option.
When it doesn't work: select All Outlook Items or All Mailboxes
So the workaround is to select any folder in your online archive, select Current Mailbox from the dropdown next to the search box, then type your search "in quotes".