He does.
You either choose to not understand it or you do not possess the capacity to do so.
There's nothing in the constitution that prohibits the states from leaving, therefore the power to do so it reserved. Quit simple.
How is it wrong?
A state like Texas could secede how every other state attempts to secede, through act or acknowledgment of the central/federal government.
How it would work:
1) Texas holds a referendum on leaving the United States by ballot measure.
2) Texas representives acknowledge the referendum.
3) A bill is put to US House and Senate over Texas secession.
4) The bill passes and is sent to the President for approval.
5) US declares Texas as an independent, and acknowledges secession.
6) Various treaties are signed to that effect between Texas and the US.
Obviously relies on three branches of government to approve it, so it isn't an easy process, but it isn't impossible either.