Try, instead, " He was all but dead when we found him"

That doesn't mean he was very much alive

The phrase 'all but' meaning 'almost' or 'practically' is long established in English.
'But' meaning 'if not' is archaic now. This usage of 'all but' seems related by descent from it. That is 'all but gone' and 'all but dead' mean 'everything
or all, if not gone/dead'.
The word is derived from an Old English word
butan meaning 'without'and 'outside' and 'except', which are three different expressions of the same idea. So it is no surprise that 'but' itself has acquired several,though related, meanings .Look:
'we never go out but it rains' [without it being the case that]
'we saved all but one of the pigs' [except]
'but for you we'd never have come' [were it not]
'he was hurt but didn't cry' [contrary to expectation; wholly exceptionally]
though what we are to make of 'my, but you're a fine looking woman !' who knows?

[Source of etymology: Collins English Dictionary]