Again, this has NOTHING to do with the praxis of Sola Scriptura.
Sola Scriptura is the epistemological embrace of God's holy, inerrant, written, knowable/unalterable, authoritative, historic, ecumencially embraced Scripture as the Rule or Canon for the evalution of positions. It's not hermeneutics and thus doesn't address interpretation at all.
What you are referring to is a common Protestant hermenteutical principle (NOT Sola Scriptura). In this principle (POORLY summerize as you indicated), the Author is assumed to be the text source for understanding a text. To put into literary terms, if I want to understand a particular theme in the writings of "Mark Twain," we can look for that same theme throughout his book and in others of his books - this, hopefully, will clarify what he means by this and what role it plays in his thinking. This is an alternative to looking at a single reference of it, declaring self to be the best understander of the theme, and then assuming that such is what he always means by that theme. It's a common tool in literary interpretation.
A common use of it actually comes from a treatice from Augustine. I forget the exact example he gave, but let's use Baptism. If we want to better understand what Baptism is, rather than looking at a single verse with a single reference to it - and then assigning self as the best one to understand what is meant, we'd gather ALL the Scriptures were God speaks of Baptism and hopefully the corpus of such will shed light on what God means by that. It's just ONE tool. No one says it works perfectly no matter what, but it can be one helpful principle.
I hope that helps.
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