English No, it is not. Not in the classical sense. Since arbitrage betting is a situation in which the player is guaranteed, with 100% probability, to exit the arbitrage bet at least without a loss, but in most cases with a good win.
Polish middles allow for an outcome which, although unlikely, can reduce the player's bankroll. Details about this are written in the article about Polish middles.
Why do we talk about them on the website? It is very simple: Polish middles use the same principles as traditional middles, when several differently directed events intersect, giving the player the opportunity to win 5-10 times more (on average, there are distortions) than to lose.
The method is not absolute in terms of reliability, but it is potentially very lucrative. And if you know how to look for Polish middles (we really do know), then you can make good money from this.