It's easily explainable even without the problems of the Soviet era with Si, though we use the Soviet Era to explain the bullshit descriptions of Si types being "lazy" or "familiar-oriented". Through JCF, we see that Si is more than what Socionics simply portrays it to be (which evidently is the aforementioned bullshit). Si is the ability to see the power and potential in ones self, ones' own physical and mental limitations, and judge realistic outcomes based on their own assessment of their own intrinsic worth, power, mental and physical attitude, etc. Therefore, the Si type is self-accepting to a point, that point ending when the Si user believes that they are actually capable of anything, such as being perfect as seen with many hard-working SLIs/ISTJs/ISTps. JCF shows us the other features of Si that others don't see (which the aspect that is seen being the "self-acceptance"/"attention to self") such as the nostalgic longing for the way things were, the associative memory through which everything in the present is compared against (something so simple as "I remember a pizza that tasted just like this back when I was younger"
). It provides a sense of knowing what one wants out of life, one's direct preferences for things, directly and consciously. There are even several more blades in the swiss army knife that is Si that I cannot even fathom due to the fact that it is my Demonic Personality Complex (8th function, most suppressed).