On a Bitcoin Whitepaper, Bitcoin is defined as a peer-to-peer(p2p) electronic cash system. The focus for today is on the p2p part and what it actually means in the context of the Whitepaper, so that when we see it anywhere we easily recognize it rather than confuse it with false or pseudo peer-to-peer.

What does it mean when a transaction or exchange is truely peer-to-peer? It should actually mean that an exchange happens while both transacting peers are in control of their funds or private keys. This could also be considered a decentralized peer-to-peer exchange, while cases where funds of one or two exchanging peers are controlled by centralized platforms are considered false p2p exchange. Again, p2p is false when one or two exchanging peers do not control their funds... the funds is/are controlled by a third party.  

So, for an exchange between peers to be truely p2p and decentralized, it has to happen between exchanging peers who control their funds or private keys rather than one or more third party platforms holding the funds on their behalf. Both the sending and receiving peers have to be in charge of their private keys.
And a two-way exchange method such as between an Instant Exchange platform and a second peer could also be considered decentralized exchange, as long as the peers (the instant exchanger and the second peer) control their keys.  Ofcourse, a better decentralized exchange should be done within a shared opensource platform that a peer can't unilaterally control, and that allows for running of nodes with smart-escrow/contract where exchanging funds are temporary held and later released to exchanging peers once term of contract is fulfilled or the right amount of funds from both peers are received by the smart contract.