Do papers tend to have more email addresses or more plus signs? I'd expect the latter, by a lot.

No, the data (as described in So's thesis) was mathematical expressions extracted from TeX source code, so the surrounding text and email addresses etc. were ignored. Skimming through by eye I can't see @ in any of So's tables, and searching for the hex Unicode value the tables list for every other character yields no hits: @ is not in the tables.

∋ is there anomalously frequently, and @ is missing, so something seems to have gone wrong, probably at multiple stages in the pipeline.