Absolutely ridiculous to say it's not conceivable that these people could be considered or treated as people

Maybe. Yet, they weren't for centuries up until 150 years ago or so, at best. Long after the signing.

as someone who feels very much in line with Quaker values

I meant to say "puritans" not "quakers" - but I don't think it makes much of a difference, and I don't know much about the particulars of different sects of christians who left Europe at the time. But, the fact remains - "America" was founded mostly by what we could call religious extremists (at the time), who were often discriminated against in their home countries, and left for "religious freedom" (win-win for the crown, too - get rid religious dissidents, maybe get a profitable colony) but this idea of "religious freedom" should not be trivialized. It's not just about what you "believe." They were extremely strict about their observances, and it dictated every aspect of their life. For instance - you may think you have values in common with them, but I doubt you dress in black with buckles every day

From what I recall, Quakers were early to publicly decry slavery and restrictions on women

"Maybe" I don't know. But in any case, the main point is this:

If you are a "constitutionalist" or "libertarian" who endorses modern globohomo multiracialism or immigration, but claim to to represent the founders intent based on the Constitution, then you are lying to yourself and others. For example, leftists say we need to restrict firearms, because "the founders never anticipated automatic weapons" or something like that. This is obviously true - but not a good reason to ban guns. Constitutionalists will decry leftists attempts at this (rightfully, based on the 2nd) - but yet, at the same time, defend multiracialism/multiculturism because it isn't literally written down in the Constitution as being forbidden, despite the fact "the founders" very clearly would have never endorsed this. Just because they didn't anticipate the ideas of "multiculturalism" or "multiracialism" or even "black citizenship" (because they were incredulous) doesn't mean they wouldn't have, had they ever known these could be things (and the historical record decisively proves this).

A nation founded by white, protestant christians that owned slaves, that clearly never endorsed black citizenship, in 1780, clearly did not intend for their nation to become a multiracial/multicultural/multireligious hellhole. Constitutionalists might be able to honestly represent the Constitution itself - but they cannot honestly represent the "founders intent." They are hiding behind a sacred piece of paper, pretending that this document alone is actually what they intended - despite all contemporary evidence to the contrary. On the other hand, we much more closely represent the intent of "the founders" - not Constitutionalists - and they will have to live with that hypocrisy (and the consequences) forever. Unfortunately, so will we