Not quite. "in" here as a prefix is not a negation thing but to _do_ something like "en" in "enhance" or "encapsulate". The word's actual latin root is "inflammare" which means to put something _in_ flames. The subject is the one doing the burning and it's transitive.
Flammable on the hand comes from "flammare", which means for something to catch fire, and is intransitive instead, i.e. the subject is the one catching fire.
The actual opposite of inflammable is uninflammable, which I reckon is only in British English at this point and mostly lost in American English.
Contronyms are what you're referring to. Indeed, flammable/inflammable, also sanction/sanction (permit/punish) and other examples such as fast/fast (going quickly/held in place).
Still, I do find "I could care less" to be less of a contronym and more of an "Americanism". I'm quite used to it by now, and shall thereby sanction its use.
Flammable on the hand comes from "flammare", which means for something to catch fire, and is intransitive instead, i.e. the subject is the one catching fire.
The actual opposite of inflammable is uninflammable, which I reckon is only in British English at this point and mostly lost in American English.