But this is an insurance company, so it is plausible that they crunched the numbers and found a real effect.
1) A website ties accounts to email addresses and doesn't allow changing them, and
2) the email provider recycles expired addresses, i.e. a new user could sign up for my old address.
I think the "functionally equivalent" part is quite arguable, at least it was in the past when many techies made the switch from hotmail to gmail.
People switched because the features and interface were far superior.
No one should use the web interface, let alone "techies." Use a real email client and everything behaves identically.
Even to this day, I don't know of an easy way to switch between fresh devices all the time and use a "real" email client.
I don't see the problem with someone preferring gmail - personally the +suffix tagging was what I still use to this day.
Say, driving a 20 year old car even though there are safer alternatives available.