Are you sure? b7 to a8 return 4.
If begin OR end position is a corner point (A1,A8,H1,H8) AND BOTH row and col are 1 step, we've a minimum of 4 moves.
This covers the moves A1-->B2; B2-->A1; H1-->G2; G2-->H1; A8-->B7; B7-->A8; H8-->G7; G7-->H8
For all other 1-1 moves, you will not fall in the second IF case and it returns 2 moves.

I play with the ASCII value: 'a' = 65, 'b' = 66... 'h' = 72; '1' = 49, '2' = 50,... '9' = 57
It doesn't matter if we move from a to b or b to a as ABS(66-65) == ABS(65-66) == 1; same for the digits.

Need extra test case...
do_test("ADAD", "BCBC", false);

ExOR approaches are passing on current test cases.

Brilliant solution, one to remember!