Just like you could come up with many explanations for what you are experiencing and misconstruing as a "handicap". Don't forget I used to defend this position. And to an extent sometimes I find myself thinking it's there. But then logic takes over.
The evidence for handicap from different people who describe it is often self contradicting. People will say "it helps to have a low rated player" others will say "it helps to have a high rated player" etc etc.
Think about it this way. What is more likely. EA actually implemented a system like this and found a way to keep their 100k Fifa 16 coins of employees in check somehow (not leaking this valuable information). Not to mention somehow testing different combinations to find which handicap produces the best pack purchases. Because you may very well create less demand for better cards by leveling out the competition.
They would have to do some intensive testing. Which again requires more employees which again is more people they would have to somehow silence. OR In reality what you perceive as "handicap" is simply parts of code that you don't understand. There is tons of behavior for the AI that we simply don't get. Because we didn't program it. It works well with your examination. Think about it.
That's because the only way you could have concrete evidence is to look at the source code of the game, which we will never get to see.
All we have to go by is experiential evidence which, according to countless players, points to the fact that the game is not really fully in our control.
Some people are good enough to overcome it (ie hectic_jukez with his insane record) so its not really an issue for them. Or, some people just don't care about the game enough or play enough to buy Fifa 16 coins when this stuff happens in game -- I assume those are a lot the people who are adamant that it doesn't exist.