The reason is because we use a layer (our main innovation) called DataSubstrate to build modular databases that support distributed transactions [1]. Because we use some MariaDB code (GPLv2) and some MongoDB code (AGPLv3) in our other projects [2] [3], so we license our DataSubstrate code to be compatible with both, and therefore, we also license EloqKV under both licenses.
I don’t think end users can pick a license because you have pieces licensed under GPLv2 and AGPLv3 . They will have to obey by both, which is tricky if they are conflicting.
My understanding is that they have a common layer over which they hold full copyright, so they can license it however they like. But since this software is also used in EloqSQL (which includes MariaDB code) and EloqDoc (which includes MongoDB code), the license has to be compatible with both. That’s why they state that the code can be licensed under either license (note the OR part).
I doubt they would accept any code that cannot be re-licensed, because the common layer essentially “transmits the infection,” for lack of a better way to describe it.
I don't think he's saying that eloqdata itself contains AGPL code from Mongo. Rather, their other project, eloqdoc, contains AGPL code from Mongo, and a user might want to integrate the two, so for simplicity eloqdata is offered under both licenses.
In that case, a user wanting to distribute eloqdata and eloqdoc together need only adhere to the AGPL, not both licenses.