As I often find, Beurei Hatefillah has an answer: Citing Machzor Vitri ch. 1:
This is the reason that we repeat the last line of Tehillim: Kol
Hanishama T’Hallel Yah Halleluya. It is based on what we learned in
Tractate Shabbos that we should be among those who complete Hallel
each day. This means that we should read the last chapters of Tehillim
each day which are the chapters that all begin with the word
Halleluya. If we did not repeat the last sentence of the last chapter
of Tehillim, we would not appear to have completed the last chapter.
We would make it appear that there were additional verses that follow.
However, now that we read the last verse twice, we demonstrate that it
is the last verse and we have completed the last chapter of Tehillim.
The English translation is at the end of the article on pp. 31-32. I recommend that you read most of the article from the beginning, as it discusses a debate on which Tehillim comprise Psukei Dezimra in the first place. Understanding that debate will provide some context on why this is considered "the end" and will provide further context on Art Scroll's comment, as well. (IMO, Art Scroll's comment could use a bit of "improvement", esp. considering that they are not exactly lacking in "space".)