I'm familiar with that sense of 是 (also as 昰) from seeing it in some obscure Japanese text, where it was used to signify これ, which was probably about right when writing was new to Japan. In Japanese, 是 is more commonly used in words that to me seem unrelated to either the old, very old, or modern Chinese use of the character.

It's amazing how durable some of this stuff is, especially if you look at it from the perspective of recipients of Middle and Classical Chinese culture (such as Japan).