That's a subtle change in wording from 5th ed. In 5th, it mattered if the unit had to test to move through terrain, not just move through it. And the change affects more than just Scarabs and other slow Swarms. Wraiths and C'Tan Shards who both ignore terrain too can now be slowed. The Wraiths, in particular, relied on slowing foes to I1 and killing them first.
"Fetch me another plaything. This one seems to have broken." -Urien Rakarth
Space Marine- "This planet is ours, witch."
Eldar- "No ... This planet is theirs ..."