Yes, this probably indicates advertising, but not all advertising is spam (both unsolicited and unwanted).
Just as significantly, as soon as a user is told that there is a mismatch, he will want to see the mismatch. When there is no mismatch to be found, he is likely to respond in ways that are not useful.
However, I think it would be very interesting to just tell him truth:
I expect users will quickly learn that some advertisers are more reputable than others.
Equally, we get a lot of spam where both addresses are for the same domain because they are using garbage domains instead of a mass mailer. But knowing that a message is advertising seems useful.
Some standardization of the 'by" address is probably desirable, to remove at least VERP and BATV. Maybe unwrap SRS headers back to the original value. Then remove anything after a "+" in the localpart of the By address. This makes the text shorter and less confusing to the user.
Of course, everyone has an opinion, so there should probably by some opt-in/opt-out choices, at least at the domain level and above.