The point isn’t whether it’s possible for secular Jews to fully keep Shabbos. The Chofetz Chaim already wrote in his preface to Mishna Berura Hilchos Shabbos that it’s impossible to keep Shabbos without learning the halachos of Shabbos.
The issue here is that we are trying to mekarev these young people to Shabbos and to Yiddishkeit. Judging from the video, I’d say it was highly successful. Is this a good way to get estranged Jews to get involved with Shabbos? Definitely. Can we consider them “Shomrei Shabbos” after just one such experience? It’s a question, but one that is not really relevant. Because even if at present they are not considered Shomrei Shabbos, we can hope and daven that they soon will become such. And what the BJX is doing is probably the best, maybe the only way to effect this change in their lives. We should be davening for their continued success.