Can an ATtiny with RSTDISBL programmed be reprogrammed over ISP?
I don't think there's any way to reprogram the AVR via ISP if the RSTDISBL fuse is programmed; perhaps the second sentence is itself referring to the high voltage serial mode? The only way I know of to low-voltage program the ATTiny with the reset pin used as an output is to use a bootloader - on startup the bootloader will check for a serial signal on its configured pins, and if it's there it will download the code via serial and then flash itself.