The above jQuery wasn’t working for replying to comments on activity, so I wrote some for that too.
It’s not the neatest and I’m sure someone could do a cleaner job, but this’ll work for the time being:
// Main Comment Box Show-er
  jQuery('.activity-content .acomment-reply.bp-primary-action').click(function(){
   var buttonId = jQuery(this).attr('id');
   var cutter = '#ac-form-'+buttonId.replace('acomment-comment-','');
  var inputter = '#ac-input-'+buttonId.replace('acomment-comment-','');
   jQuery(cutter).css('display','flex');
    jQuery(inputter).focus();
   
})
// Comment On Another Comment Box Show-er
 jQuery('.activity-meta.action .acomment-reply.bp-primary-action').click(function(){
   var buttonId2 = jQuery(this).attr('id');
   var cutter1 = buttonId2.replace('acomment-reply-',''); 
   var cutter2 = cutter1.replace('-from-\d+','');
   var cutter3 = '#ac-form-'+cutter2;
    var suffix = this.id.match(/\d+/);
   var nucutter = '#ac-form-'+suffix;
   jQuery(nucutter).css('display','flex');
    var inputter = '#ac-input-'+suffix;
    jQuery(inputter).focus();
  
})