I feel this is meant as:

"If you want to present at a big conference, then you must have experience doing smaller conferences or presentations."

You can gain experience by presenting in local meetups, making videos and posting them online, doing short tech talks at your job, etc. Doing those first will net you a lot of valuable skills and experiences.

In order to be a presenter at a conference with enough visibility that they receive six good applications for each slot, you must have already been a presenter.

For a big conference (as this seems to be), yes, definitely. Partly because conferences can sadly be quite risk averse, but also because presenting is a real skill, and assuming you're already great at it and jumping in at the deep end won't help anybody.

If you are looking to get into public speaking there are no shortage of meetups desperate to get speakers booked in, on any topic you like, in almost every city in the world (try meetup.com). Once you've done one or two you'll hopefully have some video of you usefully presenting complex ideas to an audience. From there getting into a conference gets drastically easier. Spend a little time and start with that. Having some initial visibility before jumping straight to huge international conference scale is typically a requirement, and a pretty reasonable one.

I don't want to put people off public speaking; it's super rewarding professionally and personally, and we as an industry do desperately need to hear a wide variety of ideas from as many people as possible.

It's not immediately easy though, just because you're a great developer/designer/PM/other. It's famously scary, and a quite substantial skill in itself, and if you jump from zero experience to trying to present hard concepts to an audience of hundreds of people who are paying hundreds of dollars, you'll struggle, and it won't do you or your audience or your confidence any good.

Spend a little time warming up. If you're already great, waiting a month and talking at a local meetup first will just be extra practice. If you're not, you'll want to find that out in a small friendly room of nice people, not on a huge stage in a vast auditorium live streamed around the world.

Assuming that there is no shortage of presenters, is it a stretch for organizers to pick the ones that have strong previous talks first?

For a big conference, one that attendees spend lots and lots of money to travel to and attend, the pressure on the organizers is to make sure that all the presentations (and presenters) are at some baseline level of quality so that attendees will go away happy. They are much less concerned with providing opportunities and teaching moments to aspiring conference speakers, which hopefully everyone agrees is the right set of priorities for an organizer for that type of conference.

There are all sorts of ways an aspiring presenter can get experience or demonstrate ability that don't involve catching a break from a sympathetic organizer whose job is looking out for the attendees that spent thousands of dollars to be there.

There are conferences specializing in giving rookie persenters chances. KCDC, for instance, takes chances on a lot of new people every year.

Another good way to start is user groups: Many will let anyone speak as long as they claim to have a relevant topic. After a video or two of those, you move up until you are eventually invited into conferences that invite speakers, as opposed to ask for submissions. Even for a talented speaker, the road from one end to the other will take about 3 years, speaking at least 5 times a year.

I for one don't think is worth it at all, but I have friends that have 10+ speaking trips a year.

So, in order to swim for the Olympic team, I must already have been a swimmer?

Yes. Or have a recorded talk, or a blog, or something so that they can judge you.

Really I was also about ready to smash them over the head with that requirement, but reading that part does make it seem pretty reasonable.

Presumably the bar is higher at a single track conference like the author's.

It looks like a blog would also suffice. Although I had the same initial reaction: it feels like a rather overzealous exclusion.