Is "-eth" (as in the verb "buildeth") always the singular? Is this inscription at Hoover Dam a mistake?

I was kind of surprised to find that one of the inscriptions on one of the towers at the Hoover Dam has what I thought was a typo, but I want to know if I'm in the wrong because I can't find anything online about this being a typo.

The tower says:

"Since primordial times, American Indian tribes and Nations lifted their hands to the Great Spirit from these ranges and plains. We now with them in peace buildeth again a Nation." (https://www.usbr.gov/lc/hooverdam/history/essays/artwork.html)

In the second sentence it seems that "buildeth" should read "build". Am I missing something? or is this a typo that has survived without comment for some 80 years on a very famous monument?

Sure, in Old English, "byldaþ" was used for the plural (þ being thorn, the old way to write th). A relic of this seems to even be present in Middle English (the following shows two forms though, I guess showing that the language was changing):

Furst þay [bees] bulden þe kynges hous..and þerafter þay buldeþ oþer hous.

(From John de Trevisa's translation of Bartholomew de Glanville's De Proprietatibus Rerum, a1398)

However, in the context of the quote on the dam, it looks to me more like hypercorrection, wording created by people who knew that archaic English sometimes used -eth but weren't really familiar with it. After all, Shakespeare's English is the archaic English that's typically emulated, not the English centuries before that.

In Middle English, -eth was a possible plural verb ending, as well as a third-person singular verb ending. The form of plural verbs varied by dialect. See the following page, “present tense” column “south”: Middle English Tense Inflection.

Or the verb table on this Wikipedia page.

The “th” sound is represented by the letter “thorn” there: -eþ.

I don’t know if that is the reason for the usage on the inscription.

Yes it is always singular, it is probably a mistake

-eth (source)

suffix

an ending of the third person singular present indicative of verbs, now occurring only in archaic forms or used in solemn or poetic language

buildeth (source)

verb

(archaic) third-person singular simple present indicative form of build