FTR the evolutionary delay hypothesis used to be popular, but for the carboniferous, the most carbon-deposing period on earth, that theory has been dismissed. See this paper: https://doi.org/10.1073/pnas.1517943113
TLDR:
* there is evidence for partial lignin breakdown in existing deposits, so we know it was a thing back then
* if it were just lignin breakdown, then we'd see orders of magnitude more deposits. that is, if you look at the per year deposit rate, you'll see only a small fraction of lignin being deposited.
* a large fraction of deposits doesn't even contain lignin, often below or above deposits with lignin, but without there being a different rate of depositions between them.
TLDR:
* there is evidence for partial lignin breakdown in existing deposits, so we know it was a thing back then
* if it were just lignin breakdown, then we'd see orders of magnitude more deposits. that is, if you look at the per year deposit rate, you'll see only a small fraction of lignin being deposited.
* a large fraction of deposits doesn't even contain lignin, often below or above deposits with lignin, but without there being a different rate of depositions between them.