Solution 7 B”a” is feasible but would be painfully slow.
Instead, the one point skip to B1 is tesuji.
After that, if W“a”, B“b”, W“c” captures, then the simple B“d” easily gets Black out.
So what actually happened was W2 as shown. Then after B3, W4 occupied Black’s shape point, keeping the fleeing Black group as weak as possible under the circumstances.
This was fine play by both sides, as would be expected from players at their relatively high level.