Is this possible?

Yes. It's called a whale.
$$$$$$$$$$ in chromiums

That's hard to say though. This guy has a puny roster of characters. Only two other 7 star characters. Not that many are unlocked taking in consideration how many thousands he must have spent to unlock Chirrut and Jynn 7 star. Maybe he just has a credit block and doesn't want to invest in credits.

There is no way to exploit or hack stuff like this? It Seems odd

They were added to chromiums. This is no different than any other chromium toon.

Pay to win.

Well wow, I can see why they would spend thousands to gear them up. Chirrut, Jynn, Baze, and Wiggs are a wicked combo.

I wonder how it would do in the meta....

Ugh pay to win killing the fun in our shard arena then :/ It was so fun competing for 1st till this guy...

The player could have only starred up those few characters.
When looking at other rosters you can't see how many unused shards they have for a character you only see 0/x or max, so they could have hundreds of shards for other characters and only activated and starred jyn and chirrut.

Example
My roster as seen in my account


My roster as seen by ally/opponent

Would only take around 800-1000 to come up with both toons at 7*. And as scuba points out he could have 60 other toons ready to star and has only chosen to those two.