Short answer
Dimraeth is designed to be playable solo from start to finish. Co-op is an option for groups, not a requirement to begin a character or explore the game.
What solo does not mean
Solo support does not automatically reveal difficulty tuning, companion systems, or every encounter's balance. Check the current build for the practical demands of any quest or boss.
A sensible first-session plan
Learn your own controls, class tools, and inventory flow in a personal world first. Then decide whether joining a group improves the kind of activity you want to pursue.