I guess you're right. I just see a lot of Ps around who just go and do things without thinking about them and not really devising strategies and stuff at all. I can usually point many flaws in their ideas and 'plans' with ease and they rarely have anything to say for it. Maybe just coincidence?
I will also say that there are many other contributing factors to why some of these people might be implusive, it could be the result of emotional unstability for example, and in some way's, many types can be implusive for different reasons(even Js), it's really not so simple once you think about it, I also have to make this distinction, just because someone chooses to be implusive(or just is) does not mean they are incapable of planning and thinking ahead.
I myself am a rather spontaneous(but not implusive) person, even if I plan something, I can change plans very quickly. I like just following a random idea or inspiration and seeing where it will lead, this is how I work, how think. Following some rigid structured plan from beginning to end is just not my thing at all. I just don't get the idea of INTPs as these people who make highly detailed plans, stick to them closely and quickly and decisively move to action, INTPs have the lazy "absent-minded professor" persona so it seems very strange to me that someone would consider them the most J-ish P.
That's what is going to separate INTPs from even some Ps, thinking and doing, Js are known to be doers, they want external results, INTPs love to think night and day but alot of Js don't care for that if they don't see any externally purposeful results, and I think it's known INTPs like to think, introspect, and internally analyze much more than carry out plans of action in the external world, this aspect will make INTPs look like one of the most P-ish Ps actually.