It's probably even easier to use language converter to convert existing Chinese Wikipedia into pinyin using language converter than converting between traditional/simplified Chinese so I don't get why a separate Wikipedia for zh-cmn-Latn would be needed. The standard orthography of Chinese is already very close to the spoken standard Mandarin variant and most (not all) Chinese characters only have 1 pronunciation so a simple machine conversion should be able to pretty reliably convert the existing Chinese Wikipedia into pinyin. The space delimitation between vocabulary might be a problem but that's only for good-looking/easy-reading purpose. [That is, if Chinese Wikipedia editor want to add a pinyin version to the site's language converter, which I personally don't think that is the case. Someone can still raise the question about language converter on Chinese Wikipedia Village Pump though to see how many people support the language converter proposal].

As for those existing 250 articles, I checked some of them, and many of them seems to be word-to-word, character-to-character copies of Chinese Wikipedia article that whoever put those article there didn't even mention their source in edit history or whatever. They should be deleted for copyright violation. Some will need to see if there're any content that are actually not copy-paste from Chinese Wikipedia before making decision about what to do about it.

One of the reason being stated regarding why a pinyin version of the Mandarin Wikipedia is needed was that it will help illiterate people to read and edit Wikipedia. I guess it would make more sense for Wikipedia to integrate a Text-to-speech module and expand available accessibility options to reach those audiences.