Flash Programming for AT89S2051
Where you're getting into confusion is that while ISP stands for in-system programming it often still needs a dedicated hardware programmer to provide the necessary voltages and/or protocol for programming the FLASH. For that device you'll want something such as an Atmel AT89ISP. 

I've never used the AT89 series in particular but on page 32 of the AT89S2051 datasheet you'll see it's an SPI type interface that requires a clock and data in and out lines. Some FTDI devices can support SPI communications but I'm fairly sure the PL2303HX can't and after a quick check of the datasheet it only seems to mention async mode.

Generally the easiest way would be to purchase the AT89ISP programmer or I believe there are a few DIY projects and clones available. Although unless the budget is tight it's probably always easiest to start with an officially supported programmer.