And was admitted as a state by Congress as well. And once admitted Texas falls under the same Article IV restrictions as any other state. It may split into two, or five or ten or however many, states only with the consent of Congress and the state legislature. For that matter, any other state can split into 5 states under the same guidelines.
Although it's never been tested, the JR and order of annexation stipulate that Texas can form new states with only the approval of its state legislature. Congress' approval is not required for the division. Under the agreement, those new states are legally "entitled" to admission to the Union.
To formally admit those new states to the Union, Congress would have to follow Article IV. It seems like a distinction without a difference, but the difference, theoretically anyway, is that other states require Congress' approval before they divide and new states are not automatically entitled to admission.
In the end, it matters not. Texas is not going to divide.