Personally, I suspect it was just simply a case of a small organised group hiding a Mesmer away within the objective and porting his friends inside to take out the Lord after the enemy had left (Mesmer hides away inside the tower, drops one end of his portal inside, then jumps outside and drops the second end of his portal outside where his mates can reach it to port in). All the specific objectives mentioned in the thread are towers, which are prime for this tactic, as there isn't a second wall to overcome once the Mesmer has got his team mates past the first. Keeps could likewise be a small team taking down a wall with a couple of catapaults so that they don't trigger an orange crossed swords icon to alert the enemy of their attack until it's too late.
Given the wild over-reactive accusations of the entire server being in on it and his own entire server never doing anything underhanded, it's hard to give credence to his claims of flying over walls and 1-shotting Keep Lords. More likely he doesn't understand the tactics in play against him, and rather than try to figure out how the enemy are taking objectives from him (from right under his nose, or in this case directly over top of it, it would seem), he'd rather just publicly accuse an entire server of hacking.
That said, it's worth being vigilant and not dismissing these things outright. If there are hacks or exploits in play, it's in everyone's best interests to find and report them.