Step-by-step explanation:
Let's put it this way. By "complete thought" they mean that if the words within the quotations, "like this," do not sound like a complete sentence on their own - don't capitalize it. An incomplete thought or sentence does not express a full thought. A complete sentence needs a complete thought.
Let's use the last one, for example, you wouldn't say "exciting and memorable" all on its own. It is not a complete sentence. They're simply quoting a small portion of what was said, there is no need to capitalize it.
I hope this helps!