Yeah, looking at it from a purely SEO perspective always makes decisions look kinda shady, as in they're trying to game some algorithm in order to gain more exposure. Wikipedia's biggest SEO factor has to be the massive amount of backlinks it gets. Those are purely organic and happen simply because it is generally the best authority on most any topic. It's more of a testament to how genuinely good Google's algorithm has become rather than some masterplan by Wikipedia.
Once you have 150 million inbound links, the strategy choices are easy - focus on content quality! But you have to remember what Day 1 was looking like: a tiny community, no inbound links, and a fair amount of other encyclopedia competitors trying to attract authors. Now the strategy choices are quite interesting - at the beginning of a startup you have just enough energy/runway to "kill it" in one area. Which one do you focus on? Generating the world's best content alone without the heavy lifting they've done on deep interlinking and other SEO-friendly moves wouldn't have cut it.
Further down in the comments you'll find a research paper [1] that analyzed why Wikipedia succeeded where others failed. I am not sure I bought into the conclusion (which prompted my initial comment), but at least it has a comprehensive listing of all the main players at the time:
