I agree with the manuscript in preparation part because somebody could write "in preparation" even if the project is in a very preliminary stage of completion. But submitting a publication is more substantial for several reasons. I've tried to dispel some of the notion that anybody can submit a paper elsewhere but I'll try to treat it again here. Yes, in concept anybody can submit a paper. But I find it difficult to believe (and this is corroborated by my own experience in doing peer reviews, etc.) that the absurd case usually put forth on here - that somebody would write "I love cheese" a hundred times or the ridiculous equivalent and submit it somewhere - finds any substance in reality whatsoever. I've never seen a piece of "scientific" work that laughable, although in theory it is possible. If a pre-med thinks of doing this, I would love to hear about what he or she says when asked about the substance of his or her "research" later on.
The more likely scenario is that the work has scientific basis but is not sound for some reason or has some serious flaw that leads to rejection. But this is a part of the scientific process - and it's an important part that most pre-meds never see. Most academics will have their papers rejected at one point or another - for one reason or another (more on that below). It's not the end of the world and it may not even mean that the paper is scientifically invalid. Perhaps there is a flaw they did not foresee and now they have to do the experiments to cover their ass. In fact, when many now-accepted scientific views arose, they were not favored by the mainstream. The people who came up with those ideas had to fight for them tooth and claw until they became (now) universally accepted. That's just how science works. And rejection is an integral part of the scientific process (and proof that it is working).
But papers can be rejected for any variety of reasons other than on their merit. When we're asked to review (each journal is different but in general this is my experience in the chemical world), we are asked not only about the scientific validity of the work but also whether it is suitable for the "prestige" of the journal and whether its content is broadly applicable to most of the journal's readership. So a trial of a new surgical procedure that is revolutionary and cures some cancer could be submitted to Science or Nature and be rejected not for its merits (let's assume it's scientifically sound) but because its subject matter is not relevant to the majority of the journal's readership. The authors would get a rejection notice with a suggestion to submit to a more field-specific journal.
So even though submitted papers can be rejected for many different reasons, each of those reasons offers a lesson about the scientific process and science itself. It is a valuable experience for pre-meds because it shows that even science isn't infallible. The act of submitting a paper is not the culmination of scientific work - if it is rejected, the process loops back on itself and starts over again. But the act of submitting a paper is still a monumental undertaking - reflecting months upon months of work (nobody submits "I love cheese" written a hundred times).
