I suggested the change because Shit+Mouse clashed with multiselect. I made a whole thread about it here:
It’s also explained in the release notes for this beta:
Changed the keyboard modifier for skim to Alt (Option on a Mac). This prevents Shift (previous modifier) when selecting multiple clips from changing playhead position.
I also believe changing it to Alt for this makes sense because Alt is already used as the key to move the playhead in other shortcut keys. Alt+ left or right arrow moves the playhead to the start and end of clips and Alt + [ or ] moves the playhead according to simple keyframes. So the change is actually consistent with what’s already established.