The Taz definitely frowns upon interlocking fingers, it’s unclear whether he meant only during tefillah or at any time B’es Shalom. His language, however, proves my point, which is that refraining from interlocking fingers is kabbalistic/mystical and not halachah. The mere fact that the Taz mentions it does not make it halachah. His language (?????…???? ??? ?? ????, ?????)is clearly nonhalachic, but instead kabbalistic. The terminology used is reserved for mystical consequences that supposedly were tried and true. For those who are concerned about mystical implications to seemingly innocuous practices, I would say to stay far away from interlocking fingers. But this is not halachah.
As far as the Aruch Hashulchan quoting, he could have easily quoted the Taz, chose to quote Rav Shneur Zalman, and I don’t think it matters much. But the admonition is not halachah, but kabbalah.