Doesn't 2.5% Neanderthal mean 1-40? Therefore only 6 generations back, therefore a an ancestor about 1800 was 100% Neanderthal?
It would, if there was one neander-guy, and everybody else were "pure" homo sapiens sapiens.
But if everybody ended up with 2½% neander-genes 10000 years ago, the gene pool would still be a solution with 2½ percent content today.
(provided that there was no elimination of the genes, eg sexual selection against people looking a bit neandery)