I realize that any mass can "spin" and any aircraft with a narrow CofG, CofLift relationship could be made to flat-spin, but the development of same given usual characteristics of a high-altitude full stall do not seem to me to be conducive to the development of a flat spin.
Wasn't referrig to a flat spin, a regular nose low spin would explain a lot.
Spins occur when one wing is stalled and the other is unstalled. The Bucker Jungmiester and Jungman are legendary for snap roll and spin performance and the primary reason is because both upper and lower wings are swept 11 degrees or so. Pull into a stall, yaw, increasing sweep on one side and decreasing sweep on the other. The outer unswept wing produces more lift because it is now unswept and traveling faster. The inner wing is now effectively swept 20 degrees and is moving through the air slower than the outside wing.
Visualize this same dynamic starting with 25, 35 or even 45 degrees of wing sweep. It would be almost like effectively losing the entire wing on one side. The aircraft would spin like a top.
Once entered, since the engines are underslung, power would tend to flatten the spin out. Would imagine the engines being slung off in short order, since the spin would be violent once it was well developed. Would take a turn or two to really wind up and stabilize. Then the cg would travel aft which could promote flattening.
The assertion wasn't whether it was flat or not, either type of spin could explain a lot, long as it were upright.