Does it mean with the new system someone could write 10 posts a day and then just upvote themselves?

This has nothing to do with a "new system". This has been true since HF18, when the max number of posts per 24 hours was no longer 4.

That said, if you were to write every day 10 or 20 low quality posts, you will be probably noticed and maybe flagged and your reputation would never go high.

But, it is true, even if you find this "wrong", that you could write a bot that would post an automatically generated poem every hour and upvote yourself.
If you were to do that, you would obviously not care about your reputation, but I don't see any rule currently that would prevent you to accumulate Steem Power this way.

This kind of behavior would catch the attention of the berniesaunders accounts, who would downvote you into getting no rewards. This is exactly why he downvotes now...

Indeed, you are correct. I was only thinking of flagging in term of reputation, but not about rewards.

But what if my goal was to force @berniesanders to flag me so that his voting power would decrease?

@berniesanders, this is all hypothetical, please don't flag me! ;-)

Yes it could be abused but why would someone want to ruin their reputation like that? I wouldn't do it !

No, of course, YOU would not.

But somebody who do not care about the reputation of an account that has been created anonymously might do it. And nobody would know who is behind the account.

Yes they could. But... you'd have to be evil minded to do that.. and their followers would hate them for writing 10 posts a day, everyday, doing it non-stop for weeks and months.