The words rendered "populous" and "more populous" in the NWT are the same word in the Hebrew (Strong's # 7227). The word basically means "many" or "great." It is possible to understand the first occurrence in Deut 7:1 to refer to 'many nations,' or 'great nations,' rather than 'many people' (as in population).
The second occurrence of the term is rendered "more populous" in the NWT, and could also be rendered as 'great' or 'greater' or 'larger,' rather than "more populous." (See the BibleHub translation listing here.)
But even if the word is meant in reference to population, it is also possible that the writer is speaking of these seven nations as being collectively "more populous." The NAC-Deuteronomy commentary suggests that that is one way to read the text. (p.177)
That would pare down 24,000,000 to a number larger than that of Israel, but still far less than 24 million.
One other point to note: A chronology based on the MT alone places the flood at around 2303 BCE (with variations according to who you cite). But if you factor in the LXX & SP regarding the post flood patriarchs (a 650 year increase), the date of the flood becomes ~ 2953 BCE. (For example, see here.)
The reason I point that out is because if one dates Israel's entry into the promised land at 1406 BCE (as many do), that allows for roughly 1550 years of human population growth after the flood, making a promised land population in the millions quite feasible. (For example, using this calculator, and a starting population of 8, and 1550 years, and a 1.2 % growth rate, gives an ending population of nearly 57 million.)
Of course, the 1.2 % growth rate is simply a guess. Reportedly, human population rose from ~1 billion in 1800 CE to about 7.7 billion now (220 years). This is a growth rate of about .935 %. So my guess of a 1.2 % growth rate above was predicated/guesstimated on the longer post-flood life spans reported in the Bible and more prevalent polygamy. But my point is that mathematically and linguistically, it would have been quite possible for the nations of the promised land to have (collectively) outnumbered Israel, even using a 2 or 2.5 million figure for Israel when they entered the promised land.
(Using a starting population of 8, 1550 years, and 1.3 % growth rate gives an ending population of nearly 4 billion. 1.2 % gave an ending population of about 57 million. Such is the exponential nature of population growth.)