Spreadsheets have a "table mode" which basically reduces the UI of the spreadsheet so you can't see the row headers, etc. Table mode makes a spreadsheet integrate more cleanly in to your document, but spreadsheets in "table mode" have all of the same functionality as spreadsheets in Quip including formulas and functions, etc.
To switch between spreadsheet and table mode:
1. Place your cursor in the relevant spreadsheet.
2. Open the Spreadsheet menu.
3. Select Sheet Mode.