Who built the monuments then? The ocean warriors would be considered part of the ancient builders as a race (basically they’re not villagers since they build but they can still be their own kingdom, doing their own thing) and they would likely have been around at the same time as the ancient builders who were making strongholds and such since there’s not space in MatPat’s timeline for civilizations after the ancient builders and before the Wither wipes out everyone.

So I don’t think the ancient builders making the monuments makes sense in this scenario, unless you’re to claim that they were made by an even older group of more-ancient builders, but then it seems very unlikely that the highly advanced ancient builders that built the strongholds never raided the monuments. Especially if we’re going with MatPat’s theory about them learning and experimenting with life and death and portals because the more-ancient builders could have had valuable information they would want. Basically if they were trying to learn about raising the dead I think they would have raided the monuments.

Im not sure I’ve articulated this well, so if there’s a part you don’t understand, mention it and I’ll try to explain better. My point is that the monuments would have to be older than the ancient builders, which I don’t think makes sense.