Getting back to the task at hand, building fires without matches or lighter - yes, it can be done, IF you are 1) patient, 2) patient, 3) patient, 4) patient ... see a pattern here? Oh, and a little training and experience is basically a must.
"Rubbing two sticks together" so beloved of the movies is, ummm, a bear of a way to start fires. The castaways would have had to have located a very dry piece of wood to use as the base board to create an ember, and then all the other parts (spindle, etc.), and it takes a lot of time if you don't know what you're doing, like maybe all day. And they had a lot of other things to do too.
Using a magnifying glass, binocular lens, inverting eyepiece, etc., can work fine, again, IF you have the presence of mind to get the right kind of fine, dry tinder material to focus the sunbeam on. The inside of coconut husks works well, dunno about ren trees or scaevola. It has to be very finely divided and absolutely dry.
Sparks are good (the old flint and steel from pioneer movies) if you think of sparking two wires from a storage battery together over some bone-dry tinder, or if you're really smart, and prepared, using a fine steel wool pad and a couple of flashlight batteries works great. If you have been trained to know to do that. I do not think you could get a good spark from a knife blade on coral rock.
Once you finally get a fire going, of course, the easiest way to start a new one is to Never, Ever let the old one go out! Which could account for all of the fires at the 7 site, in addition to keeping crabs at bay, it may be a reflaction of just moving the fire site around to take advantage of fuel sources or something like that. If you are already wek from heat exhaustion, not enough water, etc., dragging in firewood from all over is a pain. It's easier to just take some of the old fire to a new source of fuel.
My 2 cents as a former Boy Scout and ex-pyromaniac.