The complete MySQL API compatibility is a bold statement.
If they implemented the whole thing themselves (I would say that should take more than 4 years in a couple-of-guys setup) then maintaining that compatibility must be/will be a nightmare. If you sold me a product as MySQL-compatible and I find that some bizarre and undocumented side-effect of MySQL implementation that my code relies on is missing, you will have to emulate that side effect.
If, on the other hand, they based their implementation on MySQL code, then this raises some licensing questions (MySQL is GPL so they will have to make their changes public). They may have separated the code somehow so that this is less of an issue, though it adds a bit of uncertainty to the whole setup.
If they implemented the whole thing themselves (I would say that should take more than 4 years in a couple-of-guys setup) then maintaining that compatibility must be/will be a nightmare. If you sold me a product as MySQL-compatible and I find that some bizarre and undocumented side-effect of MySQL implementation that my code relies on is missing, you will have to emulate that side effect.
If, on the other hand, they based their implementation on MySQL code, then this raises some licensing questions (MySQL is GPL so they will have to make their changes public). They may have separated the code somehow so that this is less of an issue, though it adds a bit of uncertainty to the whole setup.