Of course I would not receive a contact request from anyone using a classic mailer with this setting, it was just my conclusion to your question.
Well, for now there certainly are some, as you know. It seems that any message received by Delta Chat undergoes the check for being a classic mail or not first. When classified as such and “All” (for classic mails) is selected, the message shows up as a contact request (unless the contact has already been accepted). But using “for accepted contacts” setting unfortunately ignores such mails sent from a classic mailer, so the next step to classify as a contact request is never performed.
If you choose “for accepted contacts” (for classic mails), you will not be able to receive classic mails from anyone else than a contact already known to Delta Chat. This can be a problem when someone tries to contact you with a classic mailer, but is not registered in your Delta Chat app, yet.