My guess, is that the internal transistor at the PG pin that pulls PG down ( de-asserts) can’t work if Vin is too low, because that transistor will have no gate drive. So in the event that Vin is low but the PG pullup is powered then you will get a incorrect PG signal.
The foolishness with the diode to the EN pin is an attempt to paper this over. So if for some reason you wish to separately power the PG pin, and that voltage is high while the Vin is too low for the PG pin to operate, BUT EN is low THEN the low EN would clamp the false PG signal to a low level. Of course if EN was high while Vin was too low and you were powering PG from an high external source, that trick won’t work.
Seems to be a really odd and convoluted concatenation of events to merit a paragraph in the datasheet.
Bottom line if Vin is too low the PG pin is going to be hi-Z. That may cause problems. Best idea is follow the datasheet suggestion and power PG from Vcc so the PG pin is only powered when the EN pin is high and Vin is valid… No false signals.