As explained under Alma 3:25, in the early transmission of the text there are quite a few places where Oliver Cowdery mixed up these and those. In each case, the critical text will follow the reading of the earliest extant source. Here in Helaman 6:25, Oliver appears to have initially written those, but then virtually immediately he overwrote the o with an e (there is no change in the level of ink flow). The critical text will maintain the these.
The earlier reference to the oaths of the Gaddianton robbers is found in verse 21 (“insomuch that they did unite with those bands of robbers and did enter into their covenants and their oaths”). So the use of “these secret oaths” is quite possible here in verse 25. It should be noted, however, that the printer’s manuscript has “those secret oaths” later on in the next verse:
Variation is, of course, possible, so we will follow the earliest extant reading here in Helaman 6:25.
Summary: Accept “these secret oaths and covenants” in Helaman 6:25, the corrected reading of the printer’s manuscript (the earliest extant source); either these or those is possible here, so we follow the earliest textual sources.