The part that makes it confusing is that all of that also applies to a stiffer open-world western RPG like Fallout or Elder Scrolls. Nobody's calling Skyrim (or more recently Starfield) an immersive sim. Half-Life often gets included and that game is completely linear and your three interaction choices for combat are "shoot with gun (including wacky woohoo gravity gun)", "whack with crowbar", or "sneak/run past".

Is Elden Ring an immersive sim?

Honestly, the defining thing that modern "immersive sims" have that "rpg shooters" don't is usually just "physics gun". Gravity gun from HL, goo gun from prey, telekinesis in Dishonored. Sure it lets you "use the environment" instead of just shooting the zombie with a bullet, but you're often just using the environment as a bullet.