I’m pretty sure they already said that the 100th episode will focus on secondary and background characters. (or something to that effect) This is just likely saying that the theme is Cutie Mark Magic (or at least the toy marketing gimmick is) and Season 5 has the 100th episode in it, which is another thing they will push in the marketing.
Don’t they mean that the 100th episode will be in S5 and the overall theme of the season is Cutie Mark Magic? I guess the only way to figure it out is to wait for the season to happen…some day.