Harry saying that Draco was Lucius’ only weak point was probably just an (accurate) surmise given the available evidence:
Lucius is well known as a hard bastard (information from random customer)
Draco is visibly well cared for (and his behaviour suggests that he is indulged or even spoiled in many ways)
Draco is being raised to be his father’s successor and equal
Therefore Lucius probably loves Draco, making him a weak point.
No secret information is actually required to make this assessment, though it might be considered a bit close to a guess.
Harry saying that Draco was Lucius’ only weak point was probably just an (accurate) surmise given the available evidence:
Lucius is well known as a hard bastard (information from random customer)
Draco is visibly well cared for (and his behaviour suggests that he is indulged or even spoiled in many ways)
Draco is being raised to be his father’s successor and equal
Therefore Lucius probably loves Draco, making him a weak point.
No secret information is actually required to make this assessment, though it might be considered a bit close to a guess.