You forget two very crucial things. 1) Even though many casuals still try to sell the idea how Chisora made Usyk’s life difficult, those understanding boxing have long said he played possum (and that he had had a pretty subpar camp complete with contracting Covid). Usyk himself confirmed these suspicions - he deliberately didn’t show his best against Chisora. Since Fury has obviously ducked him several times, I’m pretty sure Usyk knew what to do. Fury was trying to age him, waiting to see chinks in Usyk’s armour before even considering facing him. Fury knows Usyk is his downfall and he was never going to sign a fight against him unless he sees some vulnerability in Usyk. Even without knowing how Usyk deliberately showed weaker form against Chisora, one would think he was going to play possum against Dubois to trick Fury into signing. 2) The canvas was a mess. The rain was leaking onto it from that tarp that was put above the ring, and Usyk was seen several times as he tried to wipe the water off his slippery shoes. On a surface like that movement will obviously be heavily compromised.