I must admit, I do agree with this statement. It is interesting that the biggest conflict in this play is between No. 8 and No. 3, and both of these characters seem to be focusing on different objectives. Again and again, No. 3 tries to persuade the other jurors that the defendant is, in his words, "guilty." That is his biggest concern and this is something he repeats again and again. What No. 8 is trying to do, on the other hand, is see whether there is a "reasonable doubt" concerning the defendant that means they are unable to categorically state he is guilty. This is why he asks, just before the end of the play, "Does anyone think there still is not a reasonable doubt?" The way in which No. 3 responds not to the question but with his mantra, "I think he's guilty!", clearly indicates the way that he is so focused on seeing the defendant killed no matter what the evidence suggests.
However, the fact that the defendant is not convicted by this jury represents a triumph for reasonable doubt rather than guilt or innocence. The words of the Judge at the beginning of this play further highlight this:
If there is a reasonable doubt in your minds as to the guilt of the accused... then you must declare him not guilty.
The job of the jury, and the emphasis of the play, is therefore not on guilt or innocence, but on whether there is "reasonable doubt" or not.