You are being accurate but pedantic.

In Atmel circles ISP refers to the device being programmed on its pcb.
Those devices respond to SPI commands when in RESET.
You can deliver these SPI commands via a 6-wire interface which can be intelligent or a simple LPT dongle with a few Rs and Cs.

In many circles IAP refers to the device being programmed in application i.e. not in RESET. Jan and I were referring to this.

Some 8051 variants can be programmed via a USB, CAN or RS232 bootloader on its pcb.

Some 8051 variants can be programmed and debugged via JTAG on its pcb.

Most traditional 8051 variants can be programmed by HVPP (high voltage parallel programming). Yes. This always involves the chip being physically placed in a socket in an external programmer. The external programmer is expensive. You need to remove the chip from its pcb every time you program. Some chips are OTP (one time programmable)

Every method requires some sort of cable, external software, external hardware. As Eric has pointed out, the bootloader methods have probably already got the USB or RS232 hardware already on the pcb. So all you need is the PC software.

The 'usbasp' programmer in the eBay package is widely used for AVR devices. I have not seen that particular eBay one in real life. The official 'usbasp' design works fine. It only does the Atmel SPI i/f. The eBay photo does 'look' as if it has less components than the official design.

David.