That's not true, and it's especially not true for England, the primary wellspring of American culture.

The reality is a bit more complex but in one of the most hypocritical acts of history England actually had a bit of cultural myth that there were no slaves in England, and that the mere act of stepping onto English soil would free anyone that was a slave.

This was more or less legal fact, though they famously worked around with it things like indentured servants.

If you'd like to know more I'd start with

https://en.m.wikipedia.org/wiki/Somerset_v_Stewart

Which basically established that common law (traditional understandings of how society worked) outlawed slavery in England, and had basically outlawed it for as long as England has been a thing, and there was no other law to override that.

Some point to this as the start of abolition but the legal basis of the case is that it was already illegal.

One of the basises for the decision was Cartright's case. The details have been lost but this is a decent summary:

https://legalhistorymiscellany.com/2018/10/10/slavery-and-cartwrights-case-before-somerset/

So, in the 11th year of Elizabeth I's reign, a century before the Enlightenment, it was already legal tradition that there were no slaves in England and breathing the air proved you couldn't be a slave.

Obviously, the English abandoned those ideals in the pursuit of Empire, and there's a whole bunch of hypocrisy involved, but abolitionism didn't spring from the Enlightenment in England in particular, it was already there, even if they were typically human hypocrites about the whole thing.

Oh I should've said chattel slavery, but still abolitionism older than you imply. In any case, however, it was very much a thing by the time of the founding of the US and a few states had banned slavery even before the American war of independence. Even if we ignore the whole "not giving them their freedom despite the will of his father in law explicitly demanding it" part, someone like Washington could not have been ignorant of abolitionist thought except willingly.