Jonathan, that scripture does not support the assertion that hallelujah is the highest praise. It does nothing more than provide an instance wherein the command was used.
It's like Nessa said, the word "hallelujah" is actually a commanding phrase that means "you praise God." You can offer praise to anyone, for anything, but the highest praise you can offer is that to God... which is why some people insist that hallelujah is the highest praise.
Nonetheless, the word itself isn't a praise anyway, it's a command to praise. It's like saying, "clap your hands for Jesus, everybody." Except, you're saying "praise God, everybody."