I think a single line of text is actually clearer, because there's less to read and process. With two lines, people have to read both lines, understand what they mean, and then choose which option they want. With the single line of text, people just decide if they want a simple button for adding one entry at a time... or not.

[ ] Display a simple button, using the text above, for adding one entry at a time

It's like writing an essay, each time you can remove a clause from a sentence, it becomes simpler and easier to read.