To be fair, Mao Yuanyi did have some military experience (as military adviser of Sun Chengzong), although he wrote Wubeizhi before that (Wubeizhi jumpstarts his military career, more or less, but he was already well-versed ). Indeed Mao Yuanyi didn't participate in Imjin War (he was 3-4 year old when Imjin War broke out)To my best knowledge, Mao Yuan Yi was a philologist, not a general, and he never participated in the Imjin War (or in any war). His work Wu Bei Zhi was criticized by many of his contemporaries as outdated, inaccurate, and redundant.
Wubeizhi is basically a compilation of a lot of military works during or before his time. It was criticized by his contemporaries because it contains a lot of stuffs considered outdated/less useful for the battlefield of his time (or just plain bad), but for us modern readers it provide some very good info on Chinese military from Song period onward, up to that point.
As for Chaoxian Shifa/Joseon Sebup, Mao Yuanyi claimed that he got it from someone that acquired it from Korea. He never explicitly state that it was during Imjin War. Ultimately, the main argument against the Korean origin of Chaoxian Shifa is that even the author of Muyedobotongji didn't buy his origin story.