I only know two INTPs IRL, one is my brother and the other is the friend that introduced me to MBTI. The statement I'm about to make is hypothetical, I haven't done this to anybody nor do I plan on doing it.
From my observations of the INTPs and INTPc, INTPs are sickeningly passive. So much quivering self-doubt makes me want to be the EFJ of their worst nightmare just because I can and possibly to goad some type of reaction out of INTPs. I don't know why I react this way and why this particular behavior elicits almost a gut level response to INTPs from me, but it does. I don't even feel this way about ISTPs. It's the most unattractive thing and it makes me really want to be evil, full blast FE-MONSTER. INTPs seem to radiate it from the fibers of their being.
INTPs dont act too much in the external world not because of the self-doubt, but because they are content being where they are--in their inner world.
As a radical Thinking type they tend to be more confident than other types (strongest Thinking preferrence of all). They dont act out because they dont your reassurance and dont need external achievements in order to feel good about themselves.
Those who are most active in the external world tend to be the least confident and the most superficial. They take from the world because their inner being is empty, and they want to achieve by external standards because they dont trust themselves to craft their own standards. So, the EJs cant validate themselves and need for others to validate them because of this.
EFJs are the ones who tend to lack confidence, that is why they always want to act in a way that would win the approbation of others. When I asked one ENFJ how she deals with criticism--she answered that she doesnt get criticism, go figure.
EFJs would melt should they ever be at a point where others dont think that they are praiseworthy, so they always have to be on the go making sure that the whole world knows that they are good.
As Mr.Myiagi once answered to Julianne when she said something like..Let me have a belt so everyone can know that I am good.."You know that you're good and thats all that matters' he answered..--What belt do you have Mr.Myiagi..'Jc-Penny'....
So, in other words, ITPs, especially INTPs (Stronger Ti) dont need to act at all because they could care less for your approval or for external rewards because they just dont need those things to feel good about themselves. So, why bother waste energy, if you could just collect your energy from within. Its more conducive to your higher purpose (meeting the standard that really matters, not the one of other people) and it feels more natural, you dont burn your energy by being externally involved.
So, if there is one type that doesnt go lacking for self-confidence, it would definitely be the INTP because the INTP needs virtually no external support. The INTP relies on external support less than the ISTP (as you say, you dont feel that ISTPs are as passive as INTPs) because Intuition helps the INTP solidify their inner judgment in a way that ISTPs can not. ISTPs are passive as well for the same reason as INTPs, because they could care less about meeting some external standard, though less passive because their internal standard is not as firmly founded in their inner being as that of INTPs because sensation is less internally focused than intuition.