>celery seeds contain vitamin C which acts as an antioxidant that converts the nitrites into harmless nitric oxide whereas the synthetic sodium nitrite is converted into harmful nitrosamines.
This just sounds like post hoc justification. There's hardly any vitamin C in celery seeds[1][2]. Broccoli has 5x the vitamin C per 100g, despite much of it being water. Moreover, if the point is that vitamin C protects against nitrates, why not add vitamin C directly, rather than doing it in a roundabout way by adding celery seeds which also happen to contain vitamin C?