Can someone explain how rock type moves don't affect flying type Pokemon?

Question

So I had a battle at the 'Rating Battle' and fought Mandibuzz and my Pokemon was Swampert. We were both holding Rocky Helmet and this is how the battle went:

Swampert move: Rock Slide, Mandibuzz move: Taunt
(Rock Slide was super effective as usual then)
Swampert move: Hammer Arm , Mandibuzz move: Roost
Swampert move:Rock Slide, Mandibuzz move :Foul Play

When I used it here it had a normal Damage it wasn't a " super effective" then
Swampert faint then my Aggron was next I used Stone Edge and it was also not "super effective"
so can someone explain?

Comments

If Mandibuzz used Roost on the same turn you used Rock Slide and went before you did, then Rock Slide will only do x1 damage because Roost takes away the flying type for the turn.

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i swear i see too many questions like this...

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Your story doesn't live up, Roost only takes away the flying type the turn it's used, so you had to use Rock Slide, not Hammer Arm that turn o3o.

Answer 1

When using roost the user who uses roost loses its flying type for that turn. This means that Rock Slide would no longer be Super Effective against the Mandibuzz, because it's no longer actually a Flying Type anymore.

Roost

Comments

Decided to edit your answer rather than steal points for writing my own, longer one :D.