Quick look: Udongein X - Seiran DLC || Touhou Fangame Spotlight 

I think the codes may be dates, some appearing to point to certain games in the series.: The first is supposed to reference Subterranean Animism (even if it's a day off), the second is probably trying to tie itself to the PC-98 era but it doesn't appear to be a significant date (the closest title, Lotus Land Story, didn't have it's trial released until July), and the third is meant to reference Legacy of Lunatic Kingdom.

3:59 Yorihime:"I will never recover from this blemish on my name... 💀" 4:41 Touhou 19:"Yes."

honestly would have prefered to get a exclusive boss somewhere in the game. like toyohime instead of yorihime or even someone to replace junko since she's only there to appease the "junko is reisen's mommy" fans megaman x has made the bosses in the franchise be able to be balanced with both X and Zero so both have a fun time fighting the bosses. this feels less like a proper DLC and instead something that feels tacked on imo to emulate the zero playstyle.