Why is the Pedersen commitment computationally binding?
The misunderstanding you have is with the sentence "the sender is able to compute an $r'$..."

Actually, that's not true, and the "information theoretically hiding" bullet point does not state that.  What it does state is that, for every $m'$, there exists an $r'$ that satisfies the relation; however it does not imply that a real sender can find such a value.  In fact, the "computationally binding" bullet point specifically states that a real sender cannot (assuming he is unable to solve Discrete Log problems).

Instead of talking about the senders capabilities, the information theoretically hiding bullet point talks about the receiver; that is, even if the receiver had infinite computing power, he still cannot get any information from the commitment as to what is committed to (because all possible values are equally likely).  Now, the Pedersen commitment scheme is asymmetric in this (because a computationally unbounded sender can change commitments); on the other hand, it turns out that a commitment scheme between two computationally unbounded parties is actually impossible; the Pedersen scheme is as close as we can come.