How do I interpret BST/SW voltage rating vs BST/GND rating in data sheet?
When the low side FET is turned on, the SW node voltage will be close to ground as you stated. The bootstrap capacitor, connected between the SW node and the BST node, will then be charged up to about Vcc - V_diode. Once you release the low-side mosfet, the bootstrap capacitor will be "floating" on the SW node, but the BST node voltage with regards to SW will still be whatever the charge built-up during the low side on time.