An island in grid2 is a sub-island only if every one of its cells is also land in grid1.
The flood fill walks grid2, but checks grid1 in lockstep.
Crucial invariant: you cannot short-circuit with &&, or disqualified islands are left half-sunk and spawn phantom islands later.
"We explore islands in grid2, but for each land cell, we check if grid1[r][c] == 1 in lockstep. An island is valid only if zero cells touch water in grid1."
"If we used isSub && d1 && d2..., as soon as one cell fails, remaining calls are skipped. That leaves the rest of the island un-sunk in grid2, causing the outer loop to find phantom islands later!"
"Using non-short-circuit bitwise & guarantees all 4 recursive calls execute, sinking the entire component even when we already know it is disqualified."
"Time is O(m · n) as each cell in grid2 is sunk once. Auxiliary space is bounded by the call stack depth, which is O(m · n) worst case."
isSub & d1 & d2 & d3 & d4 sinks the whole component even when disqualified, preventing false positive splits.