I think there's a pretty solid argument that Google shouldn't be trusting Telus's SPF records. Just to take it to the extreme, suppose a spammer were to send emails through a gateway that they control. In that case, there would be no third-party with independent SPF records to rely on since the spammer would control them.

Google shouldn't need any "advanced algorithms" to tell that the message didn't originate from their own mail servers, or an SMTP client. I'd expect this ability from any email provider. I'd be shocked if this was possible at, e.g. Fastmail.