I think it has a symbolic purpose more than anything, and one that's only grown after seeing the end of the story. I'll just paste an idea I had about Matoro that I wrote on BZP which I think might be relevant:
"Perhaps Matoro 'lives' in an even deeper realm, a spirit state beyond the spirit state where he drifts and watches the world and its inhabitants, perhaps playing recorder/chronicler like a Ko-matoran inscribing legends and philosophies in his mind. He can't and never interacts with anyone or anything, but he's always there in some way, living out every tragic moment with our heroes, but feeling every tender sweet one too. Who knows, maybe that realm he's a part of is our realm, and Matoro is just another kid now, reading Bionicle books and comics and living every single one of those beautiful and powerful moments with the rest of us..."
So ultimately I don't know if using Matoro instead of Mata-Nui was on purpose but I like to think it was as all those hours I spent just staring at this image in awe made me feel like there was a power I didn't understand, like I was this figure (I didn't realize it was Matoro at the time) facing this strange new world with these structures and sights that are larger than life. I feel like the picture may be accidentally symbolic in a really important way.