Here the 1874 RLDS edition replaced those with these, a common error in the history of the Book of Mormon text. (For examples of where the 1830 typesetter mixed up these two demonstratives, see under Mosiah 28:1; for examples where Oliver Cowdery made this mistake in the manuscripts, see under Alma 3:25.) The 1908 RLDS edition restored the correct those here in Alma 53:18. For each case of these versus those, we follow the reading of the earliest textual sources, thus those here in Alma 53:18.
Summary: Maintain in Alma 53:18 the occurrence of those in “those young men which entered into this covenant”, the reading of all the earliest textual sources (including 𝓞).