Diablo IV uses the exact same engine architecture (a modified version of the Diablo III engine). The PS5 and Xbox Series X are just specialized computers. There is no technical reason why a solo player in a dungeon needs to ping a server to kill a skeleton. The game could easily cache the season data locally and sync later.

Blizzard has stated that the "always-online" nature is fundamental to the game's design for several reasons:

account even for those who wish to wander the wastes of Sanctuary alone. The Always-Online Architecture Unlike its predecessor, Diablo III , which eventually received an offline mode on consoles,

As of my latest knowledge update (April 2026), It is primarily an "always-online" game, even when playing solo.