Just have to wonder why a 35mm gun would be shown as that is more along the lines of what many navies equip their OPVs with. Surely a 40mm or 57mm gun would be more in keeping with the corvette designation.
Its a placeholder artwork for a powerpoint. They don't normally consult the warfighters to see if it makes sense. What it does show is this is a serious discussion now.
NATO designations separate corvette from frigate by ship size (corvette 60-100m, frigate 76-150m) and OPVs are less than 60m.
Of course the "rules" are not followed by anyone and you call ships whatever you want. I find that the rule of thumb is Destroyers - excel in all warfare domains, Frigates - excel in one domain generally good at others (ASW/ ASuW), Corvettes self defense domains only, OPV limited self defense.