So not only is it convenient that the alliance forgot to demolish the suspicious walls with mysterious ancient writing on them, now it's even more convenient that the writing has the perfect martial art that can be learned in secret, without any kind of detection. Also, someone teaching themselves swordsmanship off of some texts, without any prior experience or an instructor, is extremely suspect and questionable. If you already had plenty of experience with a sword, and then you found some texts and manuals describing new movements that you haven't seen before, then sure I could believe that you could figure them out on your own with practice, but that is only for someone who has already gained a ton of experience in sword fighting and has already sparred with opponents to have experienced real combat and reactions of opponents, to have an idea of what actually works.
Wow. Think about how convenient this is for the mc. The super duper paranoid alliance that false accused and wrongfully executed his father, a long serving and loyal soldier, then sacked the place and took every martial text, and even put an abandoned impoverished orphan under watch, saw some big walls with mysterious ancient writings on them and said "no, I know we took all the texts and are super paranoid about revenge, but let us leave these mysterious ancient walls covered in ancient writing right here and not destroy them, nothing bad could happen right?"