I would think that a coverup should always be treated as slightly worse than the thing being covered up. Because of general "don't design systems with perverse incentives" principles.
"Prosecutors said that the group launched a premeditated terror attack on the detention facility inspired by antifa ideology, by setting off fireworks, vandalizing property, and shooting at police officers who responded. One officer was struck in the neck with a bullet and survived."
Only in the case that you can prove both thing and intent to cover up thing beyond a reasonable doubt. And in that case since you've got both convictions then covering it up only needs to carry a small penalty (relative to thing).
If you don't have to prove thing then you end up with a situation where all sorts of mundane actions can be construed as covering something up.
The thing being covered up here is being in the general area when someone else committed a crime. I'm not seeing how 30 years lines up with slightly worse than that.