from what I understand 1619 had some flaws over the very complicated role of slavery in all 13 colonies. Even at the beginning not every agreed with it(slavery, not the 1619 project itself), but not everyone was willing to end it because they benefited from it economically. (so not much has changed, in that respect). Though lots of people disagree with 1619 for a lot of reasons.
I don't remember it being part of a socialist plot other than, it tends to give history from the perspective of slaves, and their importance to our country, and that black lives and contributions do matter, and all things democrat/liberal revisionist are socialist and marxist and every other anti-democratic insulting phrase you can come up with. So combined with their shoddy fact checking on the basic complicated details made it a little more "biased" than it claimed to be. Though one could argue if anything about history or journalism is really neutral... everything has a perspective, usually the victors write the history/news.
This was a first attempt to remedy that on a large scale, as opposed to a book or two here or there like "things my teacher didnt tell me" and the truth about Sally Hemings and Henrietta Lacks-- though Lacks is a much more modern example.
I don't remember splc having a problem with them, but i may have missed something. 1619 does not do nuance well, and sometimes that nuance can matter and will get flagged by splc.
It's nature of defining a dog-whistle and how much meaning is left unsaid. The whole point of saying something in a "dog-whistle" way is so that you have plausible deniability...so in many cases identifying that is like nailing jello to a wall. Sometimes splc goes too far, sometimes not far enough.
The part about SPLC is that they listed "Proclaiming Justice to the Nations" as anti-muslim hate group. And PJTN is against the "1619 Project"
But what me got started digging in on this was that "some" people were claiming that our school boards are "pushing the 1619 Project Curriculum", where all I can find is it is listed as an approved reference for classes.