An Se-young plays the Yonex ASTROX 77 PRO. Interestingly, within the ASTROX series, she doesn't go for one of the especially stiff models. The 77 PRO combines a head-heavy balance with a medium flex shaft.
That fits her game well: enough support to put pressure on from the backcourt, while still keeping a slightly more accessible, controllable feel for fast defensive actions and quick switches into the counter-attack.
There's no specific public statement from An Se-young that she chose the racket exactly for these reasons. The connection to her playing style is our own assessment.