“The Sweetheart of Sigma Chi” is a song. I think the point was to draw a comparison between the mores of the era of the song, and the general vulgarity of Emily Sellers.
Your interpretation of the Liz remarks is a stretch.
If Liz has written ‘”The Sweetheart of Sigma Chi” 1911 — Emily Sellers 2015’, your interpretation would have made sense.
It looks like she is insultingly calling Emily Sellers the Sweetheart of Sigma Chi.
And it is a lot more than a song - it is also a real person. And an honorable tradition.
Maybe she belongs to the “I Ate A PHI” fraternity.