Or because as you said, EMS Sasuke activated it upon reaching him and hadn't even reached Naruto's location when Minato activated Hiraishin. Still, none of this refutes the fact that Sasuke was explicitly shown to manifest Susano'o faster than Hiraishin execution from KCM Minato.
How are they equal in every form of speed?
Bold is irrelevant to this discussion, because that's not him outspeeding Minato's actual reaction speed.
-You say Sasuke's feat=Sasuke outspeeding Minato's reactions.
-That is wrong.
-Manga shows Minato reacting before Sasuke.
All you have is "Sasuke was able to activate Susanoo before Minato's Hiraishin could initiate" (then there's the fact that his marking was gone in the first place) not "Sasuke outsped Minato", because Minato actually reacted first from what we can see.
-Sasuke sees Naruto in danger.
-Minato sees Naruto in danger.
-Minato makes a handsign.
-Sasuke uses Susanoo after Minato is shown using the handsign.
There is nothing showing Sasuke reacting faster than Minato could. No scan of that at all.
The only other form of speed is general footspeed and they've shown no major difference from each other in that field based on their fights against Obito together, and their fights against other characters. One may be faster than the other but it's nothing worth mentioning.