You're omitting part of the story:

https://en.wikipedia.org/wiki/Death_of_Brian_Sicknick#D.C._m...

>Diaz told the Washington Post that there was no evidence that Sicknick had an allergic reaction to chemicals or was otherwise injured, but stated that "all that transpired played a role in his condition."

>Dr. Cyril Wecht, a forensic pathologist, said that Sicknick's manner of death could have been classified as accidental, a homicide, or undetermined.

It sure would be a strange coincidence if his stroke was entirely unrelated as you say.