Maritime SE Asia has to be fairly abstract because there's so little information about most regions before the colonial era. Sure the Siak Sultanate didn't exist in 1444, but the region was certainly inhabited and "civilised", and presumably not ruled by any known foreign power (though Malacca expanded in that direction a bit later). Since it hardly makes sense to create a country tag for a region we know so little about, I don't see a problem with using Siak as a placeholder to represent civilisation in the area.
Palembang, which is an uncolonised region in vanilla, was historically one of the most important cities on Sumatra, and it's very probable that it continued to be a major trading hub well into the EU period. There doesn't seem to be any record of who ruled it in 1444, and there's no name for the state that controlled it in the pre-colonial EU period. And yet given its importance as an urban center, spice producer, and trading hub it seems very strange to leave it "empty".
The thing about maritime SE Asia is that in English sources you'll often see things like "the Sultanate of X was founded in [year]", giving the impression that this represents the founding of an urban center or the emergence of a state where there was none before. The reality is that it more often represents the formal conversion to Islam by an existing kingdom, or the first recorded major power shift in a specific area.
This is what I've come up with for the region myself. Again though, there are by necessity plenty of abstractions:
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