Strongest man in the world" is hard to measure. Sure he can probably pick up a round heavy stone and carry it far, but can he deadlift 800 lbs for 2 or more reps like Ronnie Coleman?
Overall functional strength wise, yes he is one the strongest men on this planet. Can Ronnie pull a truck for a longer distance without having a heart attack? Mariusz personal bests:
* Bench press - 287.5 kg (632.5 lbs)
* Squat - 380 kg (836 lbs)
* Deadlift - 420 kg (924 lbs)
* Clean - 210 kg (462 lbs)