[homero]

They said that

One thing to note about these results is that when we get a result that says the company has a tenant, we are nearly 100% correct in that fact. However, if we say that a company does not have a tenant, we are not necessarily correct. It is possible that the google result did not point to their actual domain name, or they are using a different domain name for their AAD Tenant.

If you wanted to do this really robustly, you would probably want to get a better source for your domain names than automated google search results. You might want to also look at other combinations like “companyname.onmicrosoft.com”, however we are doing just rough estimates here.