Note that "Steel" is itself an alloy of iron & carbon. What we normally refer to as "Alloy Steels" means they have some other alloying component than carbon. Most, but not all, alloy steels have a high enough carbon content to allow through hardening. There are some low carbon alloy steels produced specifically for case hardening purposes. As the fox Chromox frames were "Case Hardened" as I understnd it I can only assume they were of too low a carbon content for through hardening, The M21's frame was a higher carbon frame suitable for through hardening. Thus while both are "Alloy Steels" they are not identical in terms of treatment. As to strength if one looks at the charts of various steels giving their strengths they will be hard put to find a heatreated alloy steel which is not stronger than a case hardened mild steel. That said though when the requirements are met with an adequate safety margin, there is little "Real Gain" acheived from a practical standpoint.
As to terminology "Heat Treat" is not a specific process. The hardening portion of case hardening is to quench from a temp above critical temp, same as on through hardening steels. While true the hardening cannot be accomplished seperate from "Heat Treating" the carburizing process can, & sometimes is, done as a seperate operation. In this process the part is brought up to heat & held for the required time in the carbon rich atmosphere & then allowed to cool slowly. The Carbon Case is thus acheived, but it is not hardened. Following this the part is then "Heat Treted" IE brought up to temp & quenched. Unles there is some specific need to do so both operations are ordinarily combined, IE the part is quenched directly from the carburizing step, even so it is basically a two-step operation. It differs from the heat treating process of a through hardening steel only in the first or carburizing step.
