In the air, anything beyond about 8 seconds is so weak that it can't reasonably be discerned from the external effect of turbulence, slight backlash in the roll circuit, slight mis-trimming, etc. So, that brush, whilst broad, is valid from a piloting viewpoint. (Well that's what they taught us as ETPS anyway, and it has served me pretty well as a rule since). 20-30 seconds to diverge, to me is as near neutral as makes no difference, and in practice will probably be overwhelmed by engine torque (but then the PA28 has a rudder trimmer to solve that).
The Bulldog is spirally stable, and very happy to spin. I am reasonably certain that the high wing Cessna's generally are too, but haven't flown one for a while so wouldn't wish to swear to it. The Easy Raider is spirally stable, but that is known to have slightly weak directional stability - it's also incidentally very hard to spin. The X'Air is neutrally stable with strong lateral and reasonable directional stability, and just able to spin.
Mild spiral stability is generally a good thing in a trainer, since (a) it gives students a get-out-of-jail card if things go mildly wrong in early training, and (b) it allows you something that can be spun. Excessive spiral stability (suffered by many WW1 aircraft) can cause an aircraft to lock into an unrecoverable spiral dive - which is arguably worse than a spin.
The relation to DR is not quite (in my opinion) as BW states. The DR ratio, like the spiral mode, is a function of the lateral to directional stabilities. Whether DR is a problem is far more a function of damping than it is of ratio; the ratio becomes most significant in DR if it is badly damped, because it helps you decide in which axis to try and increase the available damping.
G