What does numeric operations mean in a sum in sql expression?
COUNT(*)*100 in this scenario seems to be how the developer dynamically gets the percentage based on how many records are returned.  

Maybe it's better read this way:

((
    SUM(rating IN (4.5,5)) --SUM of ratings of 4.5 and 5
    /  --divided by
    COUNT(*)  --The total number of records
  )
*100) --Multiplied by 100 to make it readable as a percentage
AS pct_5_star


So if the query returns 100 records, it divides the SUM by 100 then multiplies thatresult by 100 to get an easily readable number.

if the query returns 150 records, it divides the SUM by 150 then multiplies that result by 100 to get an easily readable number.