It's not only the same heroes, there are several original heroes. Some heroes are ported and actually made better without the limitations of the wc3 engine. Clockwerk comes to mind, it is much better implemented in HoN as Pharaoh than it can ever be in wc3.
It's also a lot better than dota in many ways, it's not just graphic updates that was added to the game. Not sure why anyone would prefer dota over HoN when they claim it's the same game. The only thing dota has that is better are a few of heroes(Obsidian Destroyer, Pugna, Jakiro, Enchantress, Huskar) that has yet to be ported over to HoN.
The metagame in HoN is also completely different and always changing.
Also, it's currently free to play week if you want to try it. Sadly, you can only play no-stats games with f2p accounts to prevent stat-padding(boosting your own stats with another account). No-stats games aren't really the best place to try it since there are a lot of leavers just like public bnet dota. You still get a feel for the gameplay though.
https://www.heroesofnewerth.com/free2play.php