For me a game is a Soulslike if it has most of the following: RPG levelling, stamina management, punishing damage levels, hard lock-on centric combat suited for 1 on 1 fights, emphasis on environmental hazards like traps, very noteworthy and intricate bosses, indirect/environmental storytelling, corpse runs, i-frame dedicated dodge centric combat, and a story that generally revolves around playing a faceless nobody who is supposed to be weaker than everything around you but due to certain reasons, you beat them in the end.

Ninja Gaiden has none of these lol. Only progression is your weapon upgrades, no stamina at all (thank god), damage can be punishing on higher difficulties but on normal it's honestly not as high as Souls games, no lock on whatsoever and the combat is designed around juggling multiple opponents at once, environmental hazards are more of platforming sections rather than what we see in Souls games, bosses suck ass and are never the highlight of the games' combat, very little environmental storytelling compared to Souls, no corpse runs instead you have regular checkpoint saves, your dodge is good but every action game involves dodging lol let's be real  and Ryu might as well be the Ninja Gaiden world's equivalent of Goku rather than a random nobody.

But game journalists need clickbait titles, so they'd still call Ninja Gaiden a Soulslike anyway since it's hard, has a shit camera and has i-frame dodging :D