Its 100% overall cap so you can get up to 100% cap with just skills alone or sets/talents/mythics and hero points alone. The legend Move Speed is 50% cap or 62.5% with Epiphany rank (5). However, overall cap which can either be Legend move speed + other skills, sets, affixes, mythical, or by using everything but the legend move speed is 100%.
Basically the cap is 100% bonus move speed all around in terms of the movement gains in any form and momentum gains DMG from up to 100% move speed. At (5), basically 50*100% = 50% increased DMG.
I think it can be possible to gain more DMG from momentum even if above 100% move speed, even if the actual move gains stop but I got to test that to make sure.