The word “most” is indeed not very surprising if taken to mean >50% (as you do), but the surprising fact (admittedly not relevant for 2^64 in the quoted sentence) is that “most” can be replaced with “almost all” in the technical sense meaning “tending to 1” i.e. “1 - o(1)”, i.e. “eventually greater than 1-epsilon for any epsilon”.

Here's the multiplication table up to 9 (so for n=10 in place of n=2^64), and it already contains only 37 distinct products among its 100 entries:

     × |  0  1  2  3  4  5  6  7  8  9
    ---+------------------------------
     0 |  0  0  0  0  0  0  0  0  0  0
     1 |  0  1  2  3  4  5  6  7  8  9
     2 |  0  2  4  6  8 10 12 14 16 18
     3 |  0  3  6  9 12 15 18 21 24 27
     4 |  0  4  8 12 16 20 24 28 32 36
     5 |  0  5 10 15 20 25 30 35 40 45
     6 |  0  6 12 18 24 30 36 42 48 54
     7 |  0  7 14 21 28 35 42 49 56 63
     8 |  0  8 16 24 32 40 48 56 64 72
     9 |  0  9 18 27 36 45 54 63 72 81

That is, in decimal, only 37% of (up to) two-digit numbers can be written as products of two one-digit numbers. This fraction, which drops to 28% at n=100, only drops to 17% at n=2^64 (per the article). So it decreases VERY slowly, and it's nontrivial that it actually goes to 0.