Except its not a lie.
I've been warning about this for a long time, and so have many within Pakistan.
The fensing cannot completely stop the refugee flow, and millions WILL find a way to cross over.
There WILL be terrorist attacks. You saying that Pakistan will simply crush them won't change the fact that those attacks will happen, and lives will be lost.
No one is giggling or laughing, this is is merely an observation based on ground reality.
By the way, the US's failures in Afghanistan were 100% the fault of US politicians, not the military. The US military had multiple good plans to stabilize Afghanistan, but Bush, and Obama both gutted those plans: for example, remember the Obama era surge of troops? The US military wanted 300,000 troops over a period of 15 years, what Obama gave them were only a couple of tens of thousands, and only for a few years, because Obama was scared of the political backlash.
Anyway, back to topic, anyone who believes that the taliban taking over Afghanistan is good for Pakistan, or a US withdrawal is a Pakistani victory are living in a delusion. A US withdrawal will create a vacuum that will allow multiple terrorist groups to flourish, including the ttp. This idea that the talibam are pro-pakistan is stupid, because if they were, they would A) kick out the ttp, which they have continued to refuse to do, and B) accept the Durand line, which they've refused to do since they were in charge of Afghanistan, and in fact threatened to attack Pakistan if Pakistan ever enforces the border.
US is aware that Pakistan is fencing the border so this statement includes other contexts.
What I feel is that related parties should ask the Afghan Taliban to start next phase of their struggle that is nonviolent and pure political. Even if it requires UN peacekeeping force in Afghanistan. No violent incident from Taliban side because no actual reason remains. Plus these arrangements will help build political system taking up to elections.