Are infiltration-based classes like Eldar Striking Scorpions able to capture locked points?

No, in terms of match-wide strategy Jump Pack characters and infiltration characters serve the same role of being able to get to places without the other side necessarily knowing they were there. Otherwise they must follow the same objective rules as everyone else.

However, these characters shine when it comes to ambushing and accomplishment of secondary objectives which are absolutely vital in the more strategy-oriented Stronghold matches. For example a Striking Scorpion could sneak right in and do damage. The Jump Pack characters can do the same of course, but they’re much more obvious about it.