By tampering he becomes an accessory, which in some juridictions are punished on the same level as the principals. In this case, the principals have been convicted on terrorism charges.

fwiw he wasn't charged as an accessory in this case. He was charged with "corruptly concealing a document or record" and "conspiracy to conceal documents".

In fact federal law provides that accessorys can't be sentenced to more than 15 years if the principal crime is punishable by life imprisonment (like terrorism is).

> or if the principal is punishable by life imprisonment or death, the accessory shall be imprisoned not more than 15 years.

https://www.law.cornell.edu/uscode/text/18/3