Words mean something. When you accept (1) that if you are aware of anything at all then what you are aware of is either pleasure or pain, and (2) someone has said that their life is "free from pain," then (3) the realization that what they have said is that they are at maximum pleasure follows like night follows day.
It may be important to acknowledge that occasionally we feel something that seems like neither-pleasure-nor-pain (a kind of nothingness) but this rarely happens and also it shifts into either pleasure of pain fairly quickly (perhaps upon reflecting on it one becomes aware of it feeling painful).
Regarding "freedom from pain is the height of pleasure" - I would say this is a kind of remedy which is similar to "death is nothing to us". Without a thourough contemplation and using the mind to reason through them, then both of these do not appear to make sense - so it takes thinking in order to see the application of these two points.
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