I agree that FE probably had an impact, but probably wasn't the entire reason for doing them in. I can't imagine that there are really all that many flat earth advocates out there. But if there are, it may be the case that flat earthers are just more vocal and passionate about the cause. Geocentrists don't really seem all that willing to stand up for, or promote their cause all that much (at least it seems this way for the geocentrists on the forum here). They often prefer to rail against FE. The forum member Cassini is an exception, of course. He's quite reasonable.
If I could know that Sungenis would do an analysis that were charitable and fair to the FE side, I would read the book. But I doubt that's the case. It's unfortunate that geocentrists don't really strive to understand FE, and why its important. They just seem reactionary, as if FE were the worst thing in the world. But it can't be the worst thing in the word, since there were early Church Fathers who believed in FE.
Yes, Fathers definitely did believe earth is flat, and Scripture describes a flat earth, and quite specifically not a globe, but Sungenis and Delano either don't know about the Fathers, or they aren't dealing with the Fathers. And their objection to Scripture is incredibly weak. Sungenis says that the firmament might be flexible or airy. Well, his head is flexible and airy because the Father's teach that the firmament is not only a hard, impassable structure, but that there is water above it, not space. Any reference to the air being the firmament includes the actual firmament but describes the air along with the firmament as things above us. And what about the water above the firmament? Sungenis actually suggests that it is out in space somewhere. ??? Augustine actually speculated about the water passing over/past the stars to get to the earth and because he couldn't account for how that worked, yet he admitted that it must be so because Scripture said it. No surprise the people of today would rather believe unsubstantiated, highly questionable, pagan science than believe what God says--like St. Augustine. The pagan model is patently guilty of twisting of truth and has less credibility than anything the Holy Spirit inspired, so without proof of their model, why do people choose to believe it? Romans 3:4 Let God be true and every man a liar. As it is written: “So that You may be justified in Your words, and prevail in Your judgments.”