When davening alone from a Chabad Siddur one dosen’t say Kel Melech Ne’eman before the Shma. Is that exlusively a Chabad custom?

The Rm"a writes (61:3): "... and some wrote that anyone saying Shma alone should say 'Kel Melech Ne'eman'... and such is the Minhag. It seems to me that in any case when saying Shma with a Tzibbur one does not say 'Kel Melech Ne'eman'..." - that is the Ashkenazi custom but Spharadim don't follow this practice. The Shulchan Aruch Harav (6:4) wrote that there are those who say that Kel Melech Ne'eman is said when one recites the Shma alone. Rabbi Ro'i Margalit