Some people e.g. here (which is a footnote to this) suggest that passage might be a later addition:
- In the Vinaya (monk's discipline) the Buddha is represented as saying this, but such a prophecy involving time is found only here. There is not other mention anywhere in the whole of the Vinaya (discipline) and the Suttas (discourses). This makes it suspect as an intrusion. The Commentaries, as well as many other later Buddhist writings; have much to say about the decline of the Buddha's Dispensation in five-hundred-year periods, but none of this is the word of the Buddha and only represents the view of later teachers.
See also this answer for another example of that (this example references a detailed cross-textual analysis, which gives some insight into ways in which versions of the texts are or aren't consistent).
Or there's this answer which alleges another inconsistency (this one between a sutta and the Vinaya commentary).
It's hard to know what (if anything) to make of it. I think that suttas (including AN 8.51 which you quoted) are defined as buddhavacana or "canonical" by definition, so I'm reluctant to say it's untrue; but since you ask, perhaps you should be aware that that (and other topics tagged bhikkhuni) is part of the "context".
As for whether it's a "true" statement, maybe that's difficult to say too. It claims that ...
The true teaching will remain only five hundred years.
Pañceva dāni, ānanda, vassasatāni saddhammo ṭhassati.
... and different people have (had) different opinions about whether the "true teaching" still exists.
I suppose that arguments, for and against, tend to be based on:
- The infallibility of the canon
- A textual analysis of the canon (testing it for self-consistency, see also "four great references")
- Whether it matches people's preconceived ideas about gender and society (which in modern society might be described as "sexism" as opposed to "feminism")
I think there's been a lot written about it in this century. I gather that the Theravada Bhikkhuni lineage became extinct (a long time ago, I don't know when) which made it impossible to ordain, since a Bhikkhuni is ordained by other (pre-existing) Bhikkhuni: for further details see for example Re-establishing Bhikkhuni Ordination or for example Ajahn Brahm -- Bhikkhuni ordination.