Does this mean that the 'regular placebo' effect is purely psychological?

Umm, sure? What would a non purely psychological placebo look like? I'm not quite sure which distinction you're making

In which case could it be triggered by things we associate with certain affects but don't in fact have them? [Say if I sincerely believed lettuce had a strong stimulant effect]

Of course. Sugar pills aren't really stimulants, depressants, anti-emetics, and everything else. Keep in mind that the effects come from the alief level, not the explicit belief level.