The advice is correct , however the question as the chaps have pointed out is , whats the race goal ?
If you want to try out triathlon and see then just do it in trainers.
If you know you are already comitted you may as well crack on with proper shoes etc , power increase is in the order of 5-10%
Also how long to the event , you are going to fall off because of not unclipping at some point ... think about how youd be if this happened on race day , personally I am impervious to embarrasment which is why ive raced in black and yellow lycra....