Ruthie, did you miss the setup for question 2? It's vital to how you answer the question: "
When I'm lacking confidence, I tend to withdraw from the world by thinking..."
If you choose to use 'reliable concepts'
when you're feeling confident, then you are SJ. This means using 'reliable concepts' is your utterly normal and healthy way of functioning.
Choosing to use 'reliable concepts'
when you're feeling somewhat unconfident means that you're using it to protect and justify yourself. It's a shield, not a high-functioning tool.
According to Lenore Thomson (a type theorist), we often use our tertiary function as a justification for an ineffective course of action (or inaction). Si is the tertiary of an INxP... hence use of Si under stress/challenge is a sign that a person is an INxP.
Does the test make more sense now?
(BTW, Lenore argues that the healthy thing to do in these situations is for the INxP to try a bit of secondary Ne at this stage, to get out of the bunker they've built out of Ti/Fi-Si.)
Please note that the IxTJ's tertiary barrier is Fi. So an unconfident IxTJ falls into the trap where instead of getting out there and organising a path to deal with the world (secondary Te), they stay in the same place, saying that it's not true to their nature or it's against their values to do anything about whatever's bothering them (tertiary Fi).
You can read more about it here:
Tertiary Temptation