So according to the Black Clover wikia, Asta's swords can cut spells with its edges. Is this what happened in the battle with Sekke? Did Asta break Sekke's magnun Cannonball with strenght or antimagic?
Definitely anti magic. Sekke uses "Bronze creation magic" which would be reasonable to assume his magic balls were of the strength and toughness of bronze. Asta's anti magic blades are only able to cut magic and will be reduced to blunt weapons when used to cut anything else. (episode 17 and 18 of the anime)
I posted a youtube link of the fight as reference, but the ball is clearly cut through as opposed to bashed in and broken, so the swords anti-magic properties are what allowed him to get through Sekke's balls with ease, and not Asta's brute strength.