Yes, there are existing discussions on this in the consolidated thread --
AMEX Delta Companion certificate Consolidated Thread. The language excluding Delta One has actually been there for some time (you can look it up in the Wayback Machine), but apparently they have only recently started enforcing it (within last year).
"Companion Certificate valid for one round-trip First Class, Delta Comfort+® or Main Cabin Companion ticket with the payment of applicable taxes and fees detailed below and the purchase of certain adult round-trip fares on published routings within the 48 contiguous United States."