You’re correct. Large volcanic eruptions like Pinotubo and the more recent Tonga eruption can only cool the climate as a result if the atmospheric ash layer that they produce (tephra). This cooling occurs by partially blocking sunlight and as the ash layer gradually settles, the cooling effect is considerably diminished.

This sort of cooling effect (ash layer cooling) isn’t exactly something we should hope for or rely on, as any ash layer significant enough to cause long term climate cooling would be spread around the world and the particulate layer that blocks the sun would also cause all kinds of issues (i.e. grounding flights because the particulate matter would cause planes to malfunction).