That of mysterious causes

[Maybo]

Poe married his 13-year-old cousin Virginia, and while their marriage was happy, she died just 11 years later of tuberculosis. And the author himself died two years after that of mysterious causes - some say alcohol and drugs, others heart disease or suicide.

Source: Spooky English by EZtalk

Does “that” refer to “death”?

 

[emsr2d2]

No. The phrase is "after that". The "that" refers to the time when she died.

They got married. (It doesn't matter when.)
She died 11 years later. (She died 11 years after they got married.)
He died two years after that. (He died 13 years after they got married, and two years after she died.)