I haven't had the chance to deep dive Gunung Padang but given the region's political climate and lack of government funding for proper research, it may be a long time until we get real answers. I think the Indonesian side is happy to claim it's "pyramid" is 20,000BP as it is great for tourism. They are essentially using psuedo-archaeology to boost Indonesia's national identity. The amount invested in the site from '04 to '11 was massive, especially for exploring the fringe idea of an ancient pyramid existing at the site (there was no evidence that it was a pyramid). That said, we now know it's not a pyramid and the lower steppes were likely constructed from pre-existing basalt columns and the "mortar" claimed to have been found in the samples is natural, not man-made. The construction of the site was similar to Nan Madal which was constructed about 1000BP.
Pseudo-archaeologists like Graham Hancock always make grand claims to sell views and books. Hancock's "work" (if you want to call it that) is always riddled with inconsistencies and controversy. All we know is that the top steppes are artificial, ie man made, and can be accurately dated. That is the only known fact at this point, from my understanding. The "man made tunnels" are consistent with natural volcanic tunnels but may have been modified - that is interesting and worth exploring.
Also I think one of the main guys expounding the 20,000 BP date is an Indonesian geologist who was featured on Graham Hancock's TV show. Archaeology is not his area of expertise and his claims about volcanic tunnels and basalt "coursing" being man-made directly contradict what volcanologists (in peer-reviewed papers) claim are natural formations. This guy just thinks some of these sites are much, much older than claimed and people into alternative history are eating it up.
Indonesia is very interesting and has some incredible stories to tell. I think one day we find interesting structures underwater in Sundaland (the ocean was about 3-400ft lower around the ice age), so that whole pennisula was exposed between Borneo & Indonesia/Malaysia proper.
The builders of Nan Madol & Gunung Padang may have been apart of that second (or 3rd, 4th, 5th?) wave of what we now call Polynesian people who explored Eastward, built Rapa Nui (Easter Island) and eventually hit South America (Columbia). We now have confirmed DNA evidence of a Polynesian-South American connection. One of the leading theories at this point is that the Polynesians landed there about 800 years ago, took some wives, exchanged ideas, plants (Sweet Potato confirmed in Polynesia 800BP), language and then went back West to Rapa Nui (and some surrounding islands). Again, this is DNA evidence and is rooted in actual science. Very interesting stuff!