As per the title really, it says {{item}} isn't known in the scope of the tooltip, only in the mapped value.
This is limiting because when you have a large text value in a table and want to hover to show the expanded text you can't.
Hi @dcartlidge Thanks for reaching out about this! I believe this is because it's a header tooltip, rather than a cell tooltip.
You do have access to self, which has all of the table data:
We're tracking requests for cell tooltips as well 
It seems like we still need some way of expanding overflowing table cells in this new table component. Our team is discussing this further internally!
Thanks for this workaround @Tess I hadn't thought about accessing the parent
There's also the "cell selection" option which allows clicking on the cell to display the full contents inline, which my users seem okay with too (just in case anyone else searches for this situation later)