Yama may have known that YH could not use his full power at that time, he should be aware of the prophecy. Theres also the factor that Kubo made up YH's powers later to let him fight ichibei but from a plot standpoint he still had them before according to jugram.
Dude, it doesn't matter if Yhwach had the Almighty 1000 years ago, cause he didn't use it against Yamamoto, that is simply how it is.
And don't say Yamamoto may have known that Yhwach could not use his full power, post manga scans otherwise it didn't happen.
Yamamoto clearly says that he believes Yhwach only has basic Quincy shit and a sword. He wouldn't be lying at this point either, after all why would he?
Not only that, but the Royal Guard had all of the information of Yama's battle with Yhwach 1000 years ago, and yet knew nothing of the 'Almighty.'
Also, now that I think about it, I don't think Vollstanding existed 1000 years prior.
So, we can now connect the dots.
Yamamoto fought a Yhwach without his Schrift, Lezt Still, Spells, and Volstanding.