The 1830 typesetter accidentally omitted the word thereby in this passage. This word was the very first word on this page of the printer’s manuscript (line 1 on page 119 of 𝓟) and may have been lost because the following word (they) is orthographically similar. The 1908 RLDS edition restored the correct reading to the RLDS text.
In the Book of Mormon text, the phrase “that thereby” very commonly introduces a resultive clause (36 times), especially in the book of Mosiah (22 times). There is no grammatical motivation for deleting the thereby in Mosiah 2:4.
The tendency to omit the thereby is also found later on in the text. In this later instance, Oliver Cowdery initially dropped the thereby as he copied from 𝓞 into 𝓟:
Notice that once again the following visually similar they seems to have led to the loss of the thereby, at least momentarily.
Summary: Restore in Mosiah 2:4 the word thereby, the reading of the printer’s manuscript.