In older generations, this is true, as in the old formula (the flat one that didn't care about your Pokemon), a could be equal to 1.5 IF you were battling a trainer Pokemon. However, in new generations, the formula itself was changed, as well as a always being equal to one.
1.5 if the fainted Pokémon is owned by a Trainer
So, here's an example.
Say you are battling a level 69 (nice) chansey. Let's define the variables. I will do two seperate calculations with the only valuing differing being a.
a = 1 (in this case)
t = 1.5 (lets say you're using a traded pokemans)
b = 395 (base experience yield)
e = 1.5 (1.5 if it is holding a lucky egg)
L = 69 (nice) (this is the level)
p = 1 (if you're using a power boost or smthn)
f = 1 (affection)
v = 1 (value only changes above gen 6, this is before gen 6)
s = 1 (number of Pokemon that were in the battle)
So, by doing the formula and plugging in the variables, you get 8761 (rounded)
Now, if you put in 1.5, instead, you'll get 13141. That is a huge difference, approximately a 50% increase, which makes sense, because 1.5 is 50% more of 1.