89c51 vs 89c52
Atmel 89C51 data sheet : 4K Bytes of In-System
Reprogrammable Flash Memory
Atmel 89S51 data sheet : 4K Bytes of In-System Programmable (ISP) Flash Memory
I've emphasized in red the word
Reprogramable. Atmel didn't claimed that 89C51 are
Programable in the system (ISP), but
Reprogramable
From their point of view, the low-voltage programming mode (+5v) provides a convenient way to program the AT89C51
inside the user’s system, while the high-voltage programming mode (+12v) is compatible with conventional thirdparty Flash or EPROM programmers.
At89S51 is ISP (In System Programmable) through SPI interace. There are not ISR (In System Reprogramable).
Like AT89C51 is ISR and not ISP.
As you can notice, it's only english. And it's not stated by me. It's Atmel's statement.
How about IAP (In Application Programming) ? I'm confident you know what a bootloader means.