hi,
well, so it is another Atmel mistake: datasheet which address of I posted at the link above, says that this flag is presented in Atmel C51/C52.
It is stange: revision date of the document is May,2004. Document does indicate "80C51" and "80C52" any times so I suppose it does really describe hardware of these devices. At same time, the document shows some features I doubt they present for these chips, for example: POF, Framing Error Detector, AutoAddress Recognition, Internal BRG, ALE Disable bit, etc.
It seems, this document is just a mix of some hardware which may be used inside Atmel`s 8051s. Anyway, it is not full list - there are lack of ADC, PCA, SPI, etc...

So I am sorry for that link posted - it seems not good guide and may confuse a reader.

Regards,
Oleg