The 1852 LDS edition changed this to his in “the Lamanites might destroy all this people”, probably unintentionally. The change may be the result of the his in the preceding “to destroy his people”. The LDS text has retained the reading “all his people”, but the critical text will restore the original “all this people”.
The initial use of his people in this passage is correct since the reference is to the Lord’s righteous saints, whom the Lord will not allow the wicked Ammonihahites to destroy. Rather than allow this, the Lord prefers that the Lamanites destroy the entire people of Nephi (“all this people which is called the people of Nephi”) if they become sufficiently wicked. Elsewhere in the text, there are five occurrences of “all this people” and eight of “all his people”. In each instance, we let the earliest textual sources determine the reading.
There is one other change that needs to be mentioned here. The relative clause that postmodifies “all this people” originally read “which is called the people of Nephi”. For the 1837 edition, Joseph Smith edited the which is to who are (since the antecedent was all this people). In the 1888 LDS edition, the present-tense are was accidentally replaced by were, probably because of the were in the following conditional clause (“if it were possible that … ”). The resulting past-tense were in “who were called the people of Nephi” does not make sense, of course, since the people of Nephi have not yet been destroyed. This past-tense reading was never copied into any subsequent LDS edition because the 1888 edition never served as the copytext for any of those editions.
Summary: Restore “all this people” in Alma 9:19 since the passage distinguishes between “his people” (the Lord’s people) and “all this people” (meaning the Nephites in general, including the people of Ammonihah); maintain the present-tense verb form in the relative clause “which is called the people of Nephi” (or “who are called the people of Nephi” in the standard edited text).