getSummaryDataAsync is essentially doing the same thing as clicking "view underlying data".
If you click on a mark in desktop, you get the summary:
However, if you do this with Grand Total:
Which clearly isn't what you would expect for Grand Total. Since Tableau doesn't seem to consider the Grand Total part of the summary, I'm guessing this is why the function returns nothing.
My thought is you could do, "ignore selection" TRUE, get the summary of all the marks and total it yourself with a JS function...
Or you could always create a new sheet in your dashboard that simply has the number you want!
Hope this helps
Take a look at csv download post here: CSV Download button with Tableau JS API - meowBi
at the very end, there's a select * to .csv file but i guess you could use it to calculated sum of downloaded data and return as a single value.