As discussed under Alma 3:25, there have been numerous mix-ups in the history of the text between those and these. In each case, we follow the earliest textual sources. Here in Helaman 5:49, Oliver Cowdery initially wrote those. He first tried to correct those to these by overwriting the o with an e. But then, in order to make the correct reading clear, he crossed out the entire those rewritten as these and supralinearly wrote these. All the printed editions have followed the corrected reading in 𝓟, as will the critical text. Either reading will work here. Since there would have been no motivation to edit those to these, we can assume that 𝓞 read these.
Summary: Accept in Helaman 5:49 Oliver Cowdery’s correction to these (“about three hundred souls which saw and heard these things”), which very likely represents the reading of 𝓞, no longer extant here.