query on one reading 4

Can anybody explain this a bit more clearly (this is a part
that I just don't understand in Heidegger at all.)
Following stgpep's reconstruction
Heidegger admits that he did discover that truth is uncovering.
He, for some reason, attributed the discovery to Parmenides and at the same
time charged that Plato "forgot" the discovery itself by claiming that
the notion of truth is in some sense to be further specified a correspondence.

So far so good.
Now comes the philosophy part:
what is that the discovery discovers?

'truth' as noun is multiply ambiguous [for any need for
examples there are coutless of those]
as a predicate it seems we have a better grip on its use.
it is true that 6 - 5 equals 1
it is not true that 6 - 5 equals 78
I take it we can all agree on this.
Now comes the contentious part
someone confused or forgetful, say Plato, will claim
that '6 -5 equals 1' is true because the sentence token corresponds to
something else (say reality, the forms, ideas, plug in your preferred ontology
here if you dislike the greek ones).
What I don't see is what Heidegger contrasts with this?
what is "uncovered" when we say
it is tru that 6 - 5 equals 1 ?
or what is covered by not saying?

the doctrine itself is opaque to me.
any explanation will be welcome, since after reading the paper by heidegger
on truth absolutely nothing is said which is not this rather
idiosincratic argument from philology.


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