I believe SQLite3 uses a somewhat different implementation:

> A variable-length integer or "varint" is a static Huffman encoding of 64-bit twos-complement integers that uses less space for small positive values. A varint is between 1 and 9 bytes in length. The varint consists of either zero or more bytes which have the high-order bit set followed by a single byte with the high-order bit clear, or nine bytes, whichever is shorter. The lower seven bits of each of the first eight bytes and all 8 bits of the ninth byte are used to reconstruct the 64-bit twos-complement integer. Varints are big-endian: bits taken from the earlier byte of the varint are more significant than bits taken from the later bytes.

from https://www.sqlite.org/fileformat2.html#varint

The one you linked for SQLite4 (abandoned project) is probably a better approach. I recall that the author has said that SQLite3's varint implementation is regretful.