To clarify...
The prefix in the word 'inflammable' doesn't mean 'opposite of', but rather 'into'. Somebody is inflamed if he figuratively has a fire put inside him by words or actions that rouse him to passion. Something is similarly inflammable if it can literally catch fire. Yes, the figurative meaning came first, then the literal meaning came later.
In contrast, the prefix in the word 'unthaw' indeed means 'opposite of'. Just as a tomato can be deseeded, (to have its seeds removed), it was once just as normal to say a potato had been unpeeled (to have its peel removed) as to say it had been peeled. The prefix un- in that context is redundant. Likewise, in the word 'unthaw', the prefix is redundant.