We can use inductive reasoning to make a guess at a probable answer to your question.
Your premise:
I came to the conclusion that it must be csv
is likely wrong.
The only purpose of a CSV file is to transfer data between a source and a sink which do not understand the way in which the other stores data.
An FDR does not need to be understood by any other sink other than the manufacturer and/or incident investigators.
Adding the ability to store or retrieve data in a CSV format would add some complexity, through additional problems to solve, which is simply not needed.
All that is required is a method to extract the raw data and display it in a format that humans can understand. This is trivially done with a piece of custom software.
The designers will have no requirement to provide data in any format other than that required by the extraction software so they simply won't implement it.
CSV, and other data interchange format files, all have limitations. They lose resolution, they are inefficient in their use of resources and only exist to work around problems introduced by a lack of standards, differing interpretations of standards and attempts by commercial, general purpose software vendors to differentiate themselves or make it more difficult for systems to interoperate.
We can therefore reasonably conclude that an FDR does not store data in a CSV file. Of course, the custom software which does extract the data might contain such a feature but then again, why would it?
