1993 the 18/19 used 130mm stems. 20/21 used 150mm stems on the Kona. So steering shouldn't be to effected with 135/140 lengthening it a bit over the targeted average for that size. They where certainly not uncommon length. Your arm length and position is possibly quite different to Anthonys so would find the larger swing better over the shorter twitchy 125 length.
Still proof is in the pudding and riding it. I have 130/135's on my 17/18/19/20 frames, rigid to period suspension. The only one shorter is the newer Kona but it has longer forks on. Still its down at 120mm ('97 era). The 17" needs to be shorter as its for them wife, but for me its good.
Just because Kona themselves specified 150mm stems for the large sizes doesn't mean they don't adversely affect the handling. Fitting a long stem *always* adversely affects the handling. A 150mm is an abomination that should never be used, but in those early days designers hadn't worked out how to design mountain bikes properly and were still just adapting road technology (and road riding postures).
There is however room for variations in taste. If you find a 125mm stem short and twitchy, then your personal taste is obviously way out in the dead end of the handling spectrum - and that's fine for you, especially if you have a lot of skill to overcome it, as I'm sure you have. Most people though prefer responsive steering and that's why you won't find a stem anything like that long nowadays, even on rigid bikes.