The maps that were made specially for mania allow up to 6 keys being pressed at once.
You are referring to converted maps. These kind of maps have been converted from osu! to osu!mania by a distinct algorithm, creating a playable mania map based on various factors of the osu!standard map (e.g. where the objects are placed on the play field, hit sounds, etc.)
But since these maps are not specifically made for mania but for standard, they don't turn out to be good quite frequent. Especially on such old maps that aren't even good on the mode they were made originally for (e.g. the one you are referring to)
For maps made specifically for osu!mania and being no conversions, check this list:
http://osu.ppy.sh/p/beatmaplist?m=3&r=0&g=0&la=0
