Modulating delay time is cycling(changing) delay time by a set rate and length. XFading on a sampler which I own allows you to smooth out the loop joins. In the early days of sampling memory was expensive so , samples were short with even shorter loops which were prone to clicks.Xfading was born .True modulating of delay uses short delay times of say 1ms (phasey jet flanging) to 30ms( wide chorusing} etc.Anything above 60ms starts to exibit strong pitch changing. Using modulation on top of long delay times is a little different. My reference tying Xfade to modulation is because when I change the xfade time the character of the modulation changes which leads me to believe the xfade time affects the section of delay time being modulated in a musical way.It should be noted that time factor has a  true modulation delay, allowing chorusing, flanging and phasing. The range of the modulation is limited but thats what my ModFactor is for. I use Tap Tempo for delay time changes, usually small changes at that.. I haven't tried but maybe if you use an expression pedal to change delay time, the Xfade might smooth it all out. The end game is to find the music in all this stuff and if your like me the more options the merrier. Just my opinion, hope it helps..He who has the most toys when he dies wins!