Starting with the email, based on the content you shared, it appears to be accurate. I have some updates on this from the team. When you return to your listing's location, refreshing the 'Info for guests' details following Step 1 should get the correct test result. This will give you confidence that the tool does work.

Note: If like me you first had a typo error with your listing's router name/pass then corrected it, but still 'Start Test' remained 'greyed out', then return home and enter your home's details accurately in 'Info for guests' and now test. I believe the m-Lab speed test tool first confirms that the identity of Wi-Fi being tested matches that which is recorded in 'Info for guests' You can repeat the test several times and save the fastest! Step 2: Go to 'Amenities' / 'Internet and office' select Edit. Ensure this is accurate, respecting characters and case-sensitivity, check there's no mix-up between zeros and 'O's.

Step 1: Go to 'Info for guests' / 'Wifi' and enter your wifi router's name and pass. Here's the solution (using the Airbnb app, logged into your account whilst at your listing):

Hello guest wifi