The text says "address is not spendable by the user" rather than "address is not spendable". This basically means an address you do not control.
My understanding is that this is just highlighting that the minimum ADA you need to include is going with the token to the recipient. If you're sending it to someone else (eg. not another of your own addresses) then you can no longer spend that ADA (eg., it went to the other address along with the token).
There's a box underneath that says:
Before transferring custom tokens, users may choose to use off-chain communication to negotiate who supplies the ada to cover the min-ada-value in the output made by the transferring transaction.
So in the case that I'm sending you some tokens, we might want to agree who is going to cover the ADA that I need to also send with them.