Gomrath, do you have a link for that quote?
Whether you require a visa or not has been the subject of endless debate. And between your USCIS quote and mine from the State Department, it seems the issue is clear: You need an M-1 VISA for flight training, even if it's less than 18 hours a week.
BTW it seems the page I linked to has changed, and the "18 hours a week" reference has gone. The page now claims that the only study allowed on a tourist visa (and thus, under the VWP program), is "enrollment in a short recreational course of study, not for credit toward a degree (for example, a two-day cooking class while on vacation)."
---
Edited: Thanks to Google, it seems the quote from Gomrath is from an from an INS Memorandum from J. Williams for Regional Directors following the April12, 2002 Interim Rule. See
English USA - Issues related to study on B status in Intensive English Programs and various others, but I have not yet found a link to the INS/USCIS site directly.