Yeah, boarding is not guaranteed, but while the overall boarding chance is not increased by isolating your target ship, the chance to board that specific ship in case of boarding is brought to 100%.
From what I understand this part of the patch notes has not been updated since the boarding mechanic was fist discussed after the blogpost, though. It might have changed quite a bit in the meantime (or rather is open for future change), including the only one ship rule.
By the way, it's a capital ship (which just means grand ship), the capitol is a building in Washington, D.C. and has nothing to do with ships. I see that mistake made quite a lot.