That is custom die, or Altera don't know true dimensions of their own dies?
"Hi, I'll give 5 BTC to whoever can figure out the die size for the Altera EP3SL150F780."
"Finding this number in Altera documentation would be perfect."
I have provided Altera data you asked for.
Where?
But of course, you ain't gonna belive official Altera paper but guy micrometering unknown chip.

You aren't reading that report correctly (or you aren't convincing anyone you found an answer). That particular part of the report is detailing the solder reliability of the different BGA packages, and each line is a report of a specific test case. It doesn't list the model of die core or the FPGA series tested (or if it is even an FPGA) - it merely lists the die size of the particular chip tested in the reliability trial. This is why there are multiple die sizes for each package type listed, because many different FPGAs can go in a particular package, just like many different CPU cores can go in an AMD socket AM2 package.