Doesn't the Constitution covering admission imply de-admission?

How? In a weird way, that's like saying dying implies the ability to come back to life later - it does not, not at all. Your independence as a separate nation dies when you joing a new nation.
Further, within the Constitution, same part, is the path to sub-divide the states. So, if, say, Western Maryland can, per the Constitution, join West Virginia or Pennsylvania, that is secession of a kind, yes?
No, that's a modification of state lines. And, it takes the legislatures of the states "as well as of the Congress." So, not even close.
But, the argument in my opinion would be, what did the states think at the time they joined the union? Asked previously was whether I'd joined a party and did I think I could leave it. I certainly have joined clubs and such, and all of those had clear ways of expelling or allowing members to leave. We have none. Additionally, the Articles of Confederation called the union of the states "perpetual" - no inference needed: you are joined for good. The Constitution was written and ratified "in order to form
a more perfect union" of the states, implying the concept of "perpetual" remained.
In my humble opinion, the only way a state is able to leave is if it never ratified the constitution or asked to be entered in as a new state. Once entered in, it's in.