While I'm not going to say whether Hassan is clean or not (the association with NOP and Salazar is without a doubt highly suspicious), Engels argument is based on nothing than him liking her and being on the same team.
Being the hardest worker IS a fact that actually makes it more likely she would be doing something off the rules. Why?
Many of the "hardest working" people, like Lance Armstrong, El Guerrouj, Marion Jones, etc. were able to work so hard and put in tremendous training amounts and "superhero-sessions" (whether in winning race after race in the TdF or doing all runs at sub 5 pace like El G) BECAUSE of the PEDs. If a clean runner would do that, they would completely break down under the workload. Drugs allow these athletes to do the most volume, hardest workouts and become the "hardest workers" simply based on what they do in workouts.
Engels is literally saying that Hassan is mentally stronger than all the other elite women which is also highly doubtful. Maybe more talent, but definitely not harder working than other world-class 1500, 10k or HM runners.