In Bitcoin documentation, a mempool is a receiving node’s memory pool of transactions that the node has verified as valid but that have not yet appeared in a block.
A local view, not one global queue
The important word is “node”. Bitcoin’s P2P documentation describes a mempool from the perspective of the receiving node and warns that the result is, at best, one node’s view. Different nodes can therefore have different mempool contents.
Bitcoin’s reference documentation describes an interface that returns transaction IDs in a node’s memory pool. That is a technical description of the node’s current data, not a promise about when a transaction will appear in a block or the order in which it will be included.
What “unconfirmed” means here
A transaction in a mempool has not yet appeared in a block according to the cited Bitcoin definition. That does not make “mempool” a synonym for the blockchain itself: the mempool concerns transactions waiting from that node’s perspective, while the chain records blocks.
In short
A Bitcoin mempool is a node’s local collection of verified transactions not yet included in a block. It is not a single network-wide queue and does not establish a guaranteed inclusion time.