No one's saying that they aren't spec compliant, the writer is complaining that google doesn't tell its users that their inbox will include emails for more than 1 address.
When I get an email to an address that isn't exactly the same as mine (e.g. due to extra dots in it), desktop gmail does actually show a small informational link next to the receiving address saying something along the lines of "yes, this really is you; click here to learn more". I haven't seen this on mobile, although I use the Inbox app, so it's possible that this it's there on the vanilla gmail app