Fair play to them for going through with it and extra Kudos for doing it outside Europe.
I'm not sure I would really classify Taxis and Rickshaws as public transport, but still good work.
Not to do down their achievement, but just because it is the sort of person I am, my first thought was what was the quickest you could beat that score. I think you could visit 8 in under four hours if you focus on Brussels.
Start in WHS1: Flemish Beguinages namely Groot Begijnhof Leuven have a lovely walk through admire the architecture and after a further 19 minute stroll you could end up on Leuven's central Square gazing at the stunted tower of St Peter's church part of WHS 2 Belfries of Belgium and France.
-Walk or catch a bus to the train station 15 minutes away and jump on a regular train to Brussels Centraal, if you get the fast train it is only 22 minutes.
-When you arrive you can jump on the Metro to Montgomery station (16 minutes from Brussels Centraal plus 3 minutes for the next bit) , head above ground and walk to the second nearest tram stop which happens to be outside WHS3 Stoclet Palace,
-You can't visit the inside of the house so take in the exterior as you wait for a tram to take you 11 minutes to Oudergem-woud, the first stop inside Zoniënwoud/ Forêt de Soignes try to spot a beech tree in WHS4 Primeval Forests of Europe.
-Once that tree has been spotted retrace you tracks back to the Metro, getting off at Schuman for a short walk (tram metro and walk take 29 minutes) to Square Ambroix home of Hotel van Eetveld part of WHS5 Major Town Houses of Victor Horta.
-Again you can't see the interior so instead walk or get the bus (19 minutes) to WHS6 Grand Place (that is six World Heritage sites in just over two hours!).
Soak up the ambience with a waffle, before the short four minute walk back to Brussels Centraal station.
-Now you can sit back and relax on the train up to Antwerp Centraal, keep your eyes peeled for the huge belfry in Mechelen to complement the one you saw earlier in Leuven.
-On arrival in Antwerp admire the wonderful station whilst you get the metro/tram out to Antwerpen Olympiade where 1:15 minutes after being at your previous WHS you can see Maison Guiete part of WHS7 Architectural Work of Le Corbusier
-Again it isn't possible to visit so after a quick view, notice that it is next to the Antwerp Park and Ride so the bus can get you to Groenplaats in 16 minutes from where a three minute walk brings you to WHS8 Plantin Moretus Museum where you can enjoy a tour safe in teh knoweldge that you now hold a World Record!
Altogether the minimum time for this is (if you walk rather than run) is 3hr 37mins, with a fair wind and some robust scheduling you could comfortably beat the current record in 33% of the time, and all on scheduled public transport.
As quite a few of the sites interiors can't be visited, or are small and can be seen comfortably in a couple of minutes it isn't too different to actual proper visits of them.
So there is a base of 8 in four hours to start from, I think it would be reasonably easy to tack more on.
Heck I could even leave my office now and in five or so hours time start this having ticked off a further 4WHS in London (don't give me ideas!) that is 12 in say 10 hours not even starting from an optimal position, and not using any private transport.
Would I recommend this as the best way to visit Belgium's WHS? Nope
Would this be kind of fun to do if you have already seen them all? Yep
Is meltwaterfalls contemplating seeing if Mrs and Miss meltwaterfalls fancy a trip on Eurostar in the next few months? Perhaps :)