lax, not sure if you fully understood what ashito meant when tachibana was in an offsides position, so here's an explanation:
in soccer, teams start on their own sides of the field before kickoff. during the game, forwards often push past their side of the field, which means they cross the 50 yard line and go to their oppenent's side. the defenders of the opposing team are generally positioned in a line across the field when the game starts. the fullbacks, which are the outside backs, usually have a bit more freedom to move up the field once the game begins. this leaves one of the centerbacks to be mostly focused on defending. if a forward were to go into their opponent's side of the field, they have be wary of being offsides. the centerback of their opponent can be referred to as the "last defender." let's say the defender is standing on the 40 yard line of their own half. if the forward were to go past that defender and stand on the 30 yard line, they would be offside, because they aren't on their side of the field. to remain onside, the forward must stay on the 40 yard line with the defender in order to keep onside. the play continues if say, a player were to send a long pass to that forward, and the forward moves past the defender. this is because the forward remained on the same 40 yard line with the defender until the ball was released.
during the match, ashito was looking for someone to pass to. however, tachibana was past the last defender and ashito hadn't released the ball yet, therefore making tachibana offside. ashito said that tachibana was spacing out because he hadn't realized that he had placed himself in an offside position.
hope that helped, sorry for the long explanation lol