The wording is a little confusing there, I must admit, but the point with that sentence is that once you have RESOLVED a flipped or foreshadowed card, you don't put it back to the deck.
Normally, when you resolve an upright card, you may flip it and put it back into the deck, to facilitate foreshadowing. However, once you have foreshadowed the event, you do not put it back to the deck again. That's what the 'either type of flipped card' meant in this context. Because otherwise you could technically loop foreshadowing events forever.
For the actual flipping mechanic, I do admit that it's vague, but on purpose. The reason is because the timeline is fuzzy, you can leave an entire event hanging like that. I usually do it like you describe it, by leaving a blank space which I fill once I have chosen to connect it to something. But sometimes I might juggle the actual event further back, if it doesn't fit the current timeline of events.
I guess it would have made more sense if I had the two paragraphs (Flipping and Foreshadowing) the other way around, and instead say that if you flip an event, you write it only after Foreshadowing it with another event?
Flipping as a mechanic is older than Foreshadowing (which was added quite late), so I didn't really realize this kind of obvious connection with the two.
Edit: I'm probably going to release an updated version soon.