What they do say is that Congressional approval is needed for a state to be created in the first place, and once allowed to join Congressional approval is needed for any change in their status - combining, splitting, changing their border by a fraction of an inch. It's no stretch to conclude that by implication Congressional approval is needed to leave as well.
No, that’s not true.
The Art I section has nothing to say about that and the Art IV section SPECIFIES which situations the Congress is given power to control. Succession is NOT one of the situations enumerated in Art IV.
Your statement, “once allowed to join Congressional approval is needed for any change in their status” is not true. Only the specified situations trigger empowerment.
Amendment X tells Congress that if a power is not delegated to it by the Constitution, it ain’t a Congressional power.
It is to me and millions of others. I believe in the WRITTEN Constitution, not a living one. Nowhere is the federal government granted the power to prohibit secession.