Musicbrainz needs to take into account the post-vinyl era of EPs. As such, MB's definition is very loose. The historical timeline is roughly:
1900s, the beginning of the gramophone record:
* normal disc, 78rpm, 10/12 inches:3-5 minutes per side, one song per side. These are basically singles though I don't think they were called that until later when there was a need to distinguish. Not given a title..
* "album", a book of several discs. Common for classical music since they'd need to stretch across several discs for time.
early 1950s, the introduction of the 45rpm record, the EP, and the LP.
* "single" (generally a 7-inch 45, one song per side). Still not given a title.
* "EP" (2-3 songs per side, also generally a 7-inch 45.) -- basically a mini-album, but not always given a title.
* "album", the traditional album was still sold at this time, both in the form of multiple 78rpm discs and in the form of multiple 7-inch 45rpm discs.
* "LP", the common 12-in. / 33rpm record as we know it allowed the contents of what had previously been in book form to appear on one disc.
1960s, the "A" side
By this time the multi-disc "album" had been pretty much entirely replaced by the LP and 78rpm was basically dead.
* "single" (pretty much only 7-inch 45rpm, one song per side). Now these were given titles, that of the more prominent "A-side" song, with the B-side being a more obscure or less-promoted "secondary" song.
* "EP", as above but now they were more often given titles. Still mostly 2 songs per side for a total of 4 songs. More solidly a "mini-album". Sometimes sold as multi-disc releases as a "double EP". Lots of confusion as to whether the EP is the physical format or the logical release sold on that format.
* "LP / album", same as above.
1980s, the CD single
* "single" extends beyond 2 tracks -- still has a much more prominent "single" song but might have multiple secondary songs. Very common to have the traditional 2 songs and include lots of remixes of the "main" song.
* "EP": Since at this point there is little real difference between an EP and an album, logical definitions were made. According to wikipedia, the RIAA says an EP is "3-5 songs, under 30 minutes" while the UK definition says "4 or fewer tracks, under 25 minutes". At this point the physical format is basically irrelevant to the concept of an "EP". Many EPs have "EP" appended to the title to justify the shorter length.
* albums are still the same thing but now sold on CD and start to get a little longer.
1990s and beyond: digital
Since digital media can be any length essentially without regard to physical format, the distinctions are mainly down to tradition. Lots of artists love to push the boundaries of these definitions in one way or another. "EP", "album", and "single" basically mean whatever the artist intends it to mean, and the RIAA/UK chart definitions above are less relevant.
So you can see that "EP" has changed a lot over time, making a complete definition difficult without being fairly specific about the era.