If the SEC were to invite any school that is a member of another conference, they could be sued for tampering. A&M must officially leave the Big XII and then apply to be a member of the SEC. The statement released by the SEC is merely to cover themselves from a legal standpoint. The SEC still wants into Texas, so they will take A&M if/when they apply.
A&M could have to pay $30 million to exit the Big XII. They will argue that somehow the Longhorn Network invalidated their agreement and then they would not have to pay.
The other interesting complication is that ESPN has a contract with both the Big XII and SEC. They have a huge conflict of interest. If A&M goes to the SEC, ESPN must renegotiate the payout, per their contract with the SEC. If the Big XII drops below 10 members, it invalidates ESPN's contract. ESPN could be seen as inducing A&M to go to the SEC in violation of their good faith clause in their contract with the Big XII. Here is an article explaining things much better than I could.
http://outkickthecoverage.com/how-espn-is-complicating-am-to-sec-deal.php