Certainly looks like foam mastik’d to the ceiling. I think I’d wooden batten, fill voids with foam and plasterboard on ‘top’. And, as you say, thicker underlay below whatever floor covering is above.

More worrying is why they’re bothering at all. If they’re parking a car in a single garage, they’re not using it as a workshop so what are they doing in there that needs soundproofing? If they’re just sitting in there with the engine idling, they’ve got bigger problems…