I had the same reaction originally though, because I feel like I had seen this previously as just “bending” the list of 1-100 in half.

1+2+3+4+...+49+50
100+99+98+97+...+52+51
=
101+101+101+...

101 * 50.

So you have to do a bit more thinking to define your equation but the equation takes you straight to S instead of 2S.

And since the meme just has + … instead of showing where the end of the list was, I see how one could easily mix up the 2 approaches.