Imputation of ancestral sins is a hyper-covenantalism not held by neither baptist theology nor covenant theology.
Say someone joins the PCA tomorrow. If he guilty of sins committed by people in the PCA 50 years ago? Say that person leaves the PCA in five years time and joins the SBC. Is that person now guilty of sins committed by people in the SBC 50 years ago? Further, does his guilt for past PCA sins fall from his shoulders the moment he leaves the PCA? Lastly, does the repentance just performed by the SBC and PCA acquit all future members of said guilt and sin?