I don't think SeaBIOS should be altering coreboot information. (Doing so leads to all sorts of painful debugging problems, for example.)
Well, currently it is marking at least first couple of kilobytes of memory (4 if I recall correctly) as free to use RAM. There coreboot tables are located, or at least a pointer to tables in higher memory. Because of that these tables can get overwritten by SeaBIOS or OS that starts later.
Having a device marked as an available RAM is even worse - writing unchecked values to some device registers (e.g. by a coreboot-aware system which makes use of outdated memory tables) can lead to undefined behaviour (best case scenario) and even physically damage the platform or connected devices. Trying to debug this can be even worse, especially with address space randomisation implemented in most modern operating systems.
This patch affects not only memtest, it is just clearly and immediately visible there.