The game is decent enough, but it certainly is not an "idle" game. While there are such things as "idle" builds (i.e. your DPS increases when you don't click) I still wouldn't consider it an idle game. 

The primary reason for that is that you can't really leave it alone, and let me explain why. 

I define "idle" as being able to leave the game unattended (usually closed) and seeing a lot of progress when you come back. I take the strictest definition of "idle"--not having to do anything. Take adventure capitalist for example--you can leave the game for a day, come back the next day and find you have a number of upgrades to get, or at least you can ascend for some angels. 

In this game, the "prestige" system is called Hero Souls, and each HS increases your DPS by 10%. Great right?

Except it's impossible to get HS by idling. You get HS for every 5000 levels of your heroes, or by defeating "primal" bosses (random bosses that drop HS, the higher the level the more HS they drop). 

However, you do not progress while you are idling. If the game is closed, you do not progress at all. You simply get the money for killing monsters at your level. But if you close the game at say, level 149, you will come back to the game at level 149. 

Which means you need to progress the level by yourself in order to get your HS. 

However, there is "progression" mode, which only works when the game is open. This is a definite boon, but it still isn't sufficient. Progression mode simply gets you to the next level when you defeat the required number of monsters. Higher level monsters gives you more gold. However, this is more of a mere convenience than it is a way to really "progress," because the hero levels you have when you progress at say, level 150, will be insufficient 10 or even 5 levels away due to exponential increase of monster HP. 

That's perfectly fine, but that means you need to actively babysit the game and level up your heroes when you have the gold for it. This is not my idea of idle. 

"Well, can't you just close the game and wait a few hours and come back and level up all your heroes enough to beat level 160??"

Yes and no. Because hero costs increase exponentially, the gold you get at one level is a paltry sum compared to the gold you get at 5 levels later. So a day's worth of actual idle might let you level up your hero 100 times (which is pretty much the lowest common denominator for leveling heroes), which can get you past maybe 5 levels, but it would take maybe 5 minutes to plow through those 5 levels with skills (and by buying the hero levels as they become affordable). 

It's the nature of the game to be like this, but don't consider this an "idle" game. It's simply a clicker game with some idle features.