The second occurrence of the preposition by seems strange here. One wonders if some other preposition (such as with or in) might be the original one. In other words, perhaps the second by is an accidental repetition of the first one.
On the other hand, the second by can be interpreted as meaning ‘by means of ’. There are a few examples of this meaning for by elsewhere in the text:
For the first example, the 1830 edition replaced the by with the preposition with (see the discussion under Alma 27:24). On the other hand, in Alma 58:8 there is example where 𝓞 read with, but that preposition was replaced with by (with the meaning ‘by means of ’) when Oliver Cowdery copied the text from 𝓞 into 𝓟 (for discussion, see under that passage). The occasional occurrence in the text of by with the appropriate meaning ‘by means of ’ argues that no emendation should be made here in Helaman 9:6.
Summary: Accept the phrase “by a garb of secrecy” in Helaman 9:6 since the use of by with the meaning ‘by means of ’ occurs elsewhere in the text.