Maybe he didn't. Gus had good reason to tell the Salamancas it was Hank whether he knew that to be true or not.
Gus told the Salamancas that Hank killed Tuco, but there is no actual evidence that he really knew that to be the case. In addition, there was a strong motive for Gus to tell them it was Hank whether he knew it to be true or not. Consider what Gus hoped to achieve...
Gus's approval for the Salamancas to attack the DEA agent who killed Tuco, and his identification of Hank as that agent, created a situation in which a number of positive outcomes (from Gus's perspective) were either guaranteed or highly possible. They were:
- Walt--whom Gus needed--is protected by focusing the Salamancas' desire for vengence elsewhere. Achieved - The Salamancas forgot about Walt and seemed to prefer going after the DEA agent.
- The attempt by the Salamancas combined with Gus's warning to Hank created a messy and very dangerous situation for the Salamancas in which they could have been killed, severely wounded, arrested, or identified. Even if none of these things happened, they would still need to leave the US for a long time (perhaps permanently) or at least have their activities in the US hamstrung by the need to avoid law enforcement. Achieved - one Salamanca dead and one maimed, arrested, and left an easy target for assassination.
- If the Salamancas were discovered to be the culprits, it would vastly increase the pressure that US-Mexican law enforcement would put on the cartel, crippling the powers that were threatening and controlling Gus. Achieved - the cartel bosses were shown bemoaning this exact outcome.
- Hank, who poses a danger to Gus because of his close relationship to Walt, was probably even more likely to be killed or severely wounded than the Salamancas. Even if not killed, a crippling wound could end his law enforcement career. Almost achieved - Hank nearly died and almost had his career ended by his wounds.
Neither Gus's overall gambit nor any of the calculations or machinations that went into it actually required him to know for a fact that Hank killed Tuco. Even if we assume that he did not actually know that Hank killed Tuco, it makes perfect sense for him to tell the Salamancas that he did.