He’s referring to people when they were Republican, not before their conversion. There probably were former slave owners who became abolitionists and joined but it likely wasn’t common.
It’s an easy call since the reason the Republican Party was founded was opposition to slavery [it was founded in 1854] was to oppose the Kansas-Nebraska Act and eventually to abolish slavery altogether. There would be no reason for a slave owner, even if a Whig, to join the Republican Party, which at its beginning included free blacks.
It might be possible that a free black Republican or abolitionist might have bought slaves in order to keep families together since it was not uncommon for slaves on different plantations to be married and not uncommon for freemen to be married to slaves. But it would not be for labor, rather, to try to save earnings in the hope of buying the freedom of a spouse or child. Abolitionists might try to reunite people too. But while legally the buyers ‘owned’ the purchased, having paid fair and square, or being in the process of paying, obviously in practice they were not holding slaves.
In the last several years I have learned enough of the subject to wonder if their motives were altruism as we have been led to believe, or something more sinister.
I keep stumbling onto disquieting bits of information about how things happened, and one of the things i've learned is you cannot always trust what people say are their reasons to be the real truth.