The previous poster didn't say "We can assume they have arrested the correct person", they said (in effect), "If we assume for the sake of argument that they have arrested the correct person". Your point is absolutely valid, but entirely separate from this discussion of Miranda.

I'm assuming it only for the purposes of making my argument here on HN, not for determining the guy's guilt or innocence. I have to make some assumptions because I haven't seen any photographs or any evidence myself, so I have only a 3rd hand opinion based on news reports. The people who arrested him, by contrast, are in a position to identify him as the person they saw shooting at them earlier.

If I was sure they had arrested the right guy and that he was guilty of all those things, I wouldn't have put the 'assuming...' in there at all, but just stated what came after as fact.

To be fair, this was probably just a matter of phrasing on his part. Instead of using the word "assume" (triggering a pithy response) he should have stated this:

    If the man the police have arrested is indeed 
    Dzhokhar Tsarnaev, the man implicated in the 
    bombings...