Google-owned properties get preferential placement in search results. Google-owned properties don't pay the DMCA penalty(note). Google-owned properties get treated as safe automatically. There are all sorts of ways Google uses its position as the largest search engine to promote or give preferential treatment to its own non-search properties, and this is nothing new.
(note)Google will penalize a site's ranking in search results if Google receives large numbers of DMCA takedown requests for search results on that site. Except, of course, if the site is a Google property, in which case no penalty because Google knows its properties follow good procedures for handling DMCA complaints. Good luck convincing them your site does.
Google's product, Gmail offer considerably lower security and privacy than this competitor and using it instead of ProtonMail may very realistically get someone (a journalist, whistleblower, LGBT activist...) in deep trouble or even killed in lots of places and contexts.
I think this is a pretty evil move. Not only for this but what it represents when, again, millions of people were educated for years to enter the Internet through this search engine and don't have any clue that it may not only tamper with the order of the results but also hide websites.
Anyway it is, for most cases I can think of, much better than Gmail.
So what's the theory, Google waited and waited, watching and hoping that protonmail would change their address, to give them cover to change the ranking manually?
But... even if the worst is true, and Google treats its search results like its own personal pulpit, is that so bad? Should Google be required to advertise for all its competitors in every field, just because they happen to have a website?
Google's algorithm is secret; there's nothing to imply that its results are anything except editorials.
Plenty of things are unpopular, or illegal, or annoying but not necessarily immoral.