I think the release in the same month Sekiro launches has more to say than we are willing to admit.
Anyway, good to hear sales aren't that low.
DMC is a well established franchise.
Sekiro is a totally new game, coming out of Bloodborne creator.
I really doubt it cross path. People who like Soul/Bloodborne type of game is quite different from DMC. If anything, DMC share a lot more with Bayonetta, MGR, and to certain extend GOW. People who care about all action game will buy both anyway. Character action genre has quite a small audience, and they seems to be dedicate to every game that come out. I can't imagine these groups of people will decide to skip DMC V just because of Sekiro.
IMO, it's just the decline of the genre and franchise in general. Not everything has to be competing. DMC V will still sell exactly the same in the month with no other action game.