That's not what the Horseshoe theory proposes. It suggests that the tactics used by both sides are similar, which is often true, and that either ideology could appeal to the people that get involved with far right and far left groups which is the part that is completely unproven and is absolute bullshit.

The guy who joins ISIS because of how their views of Islam are not honored who thinks the degeneration of society is due to the movement away from Allah is not going to become a communist under different circumstances much like a Marxist is unlikely to adopt a pro-monarchist POV.

Again Horseshoe theory is bullshit but your source says nothing at all about it.