Ya, it's a dumb argument. I'm not sure I made this point at the time because the whole end of semester paper frenzy was underway but I was recently reminded of it.
First, let me note what I've said before that locking down the whole city was overkill and pretty stupid IMO, so lets stick to Watertown and the environs.
First and foremost, for the more strenuous argument that I saw in some places that this showed how worthless government was, I'll just point out that the owner of the house didn't exactly seem interested in going in the boat without knowing what Tsarnaev had in there and grabbing him. The police did that.
But the less strenuous point that the lockdown was the problem seems absurd too. If there was no lockdown of Watertown do you think Tsarnaev would have run to a local boat and just hid there? Of course not. He might have been heading on to NYC (or for that matter Boston) by the evening. He might have just gotten away, but he also could have set off more explosives. There seems to be general agreement except for among the usual suspects that there was nothing unconstitutional about the lockdown (even if its breadth was ill-advised). They're wrong when they say it was ineffective too.
You should always be thinking in terms of counterfactuals. And if you don't like my counterfactual, fine - that's something worthy of argument. But don't point to an observation and think you've performed an impact analysis.