Can a torrent with zero seeders download?

DHT and Peer Exchange are not tracker related, but program related, so yes they can find people who are downloading the same file but are not connected to the tracker you are using. Those people that are found might be either seeders or leechers.

From this BiTorrent FAQ:

When there are zero seeds for a given torrent (and not enough peers to have a distributed copy), then eventually all the peers will get stuck with an incomplete file, if no one in the swarm has the missing pieces.

When this happens, someone with a complete file (a seed) must connect to the swarm so that those missing pieces can be transferred. This is called reseeding. Usually a request for a reseed comes with an implicit promise that the requester will leave his or her client open for some time period after finishing (to add longevity to the torrent) in return for the kind soul reseeding the file.

Yes, it is true in practice - but not always. With no seeders, there's no guarantee that 100% of the download is available.