If you fell a Boxer, he has no techniques in his Boxing repertoire to counter you

Avoid an opponent's strong points and attack his weakness. This is an excellent tenet in combat. Covering a Boxer will avoid his strong points as it prevents him from striking, felling him is attacking his weakness as he has no techniques in his Boxing repertoire to counter.

This does not necessarily mean you will beat the Boxer. If he is very skilful, you may be unable to cover him and fell him, and he will strike you. If you know this tenet and practice it well to attain reasonable skills, you will be in a better position to handle a Boxer. But if you just free spar without systematic training, you will still be at the mercy of a Boxer no matter how long you free spar or practice set demonstration.