That seems iffy.
But wasn't the emancipation proclimation an executive order ?
At the time of the proclimation there was no constitutional support for an ending of slavery.
The proclimation at first only freed the slaves that were in rebellious states, of course that is where most of them were.
So it was not universal , it freed a class of persons from ownership, with a geographical limit.
The emancipation procllamation is iffy constitutionally , but because its effects were found to be so positive , this gets forgiven and forgotten.