Hopefully you see how some people calling it a sonic clone is dishonest and misleading. They are two different beasts.
It's not a Sonic clone entirely but it shares a lot with Sonic in terms of a lot of gimmicks and design choices in the stages. I'd say Sonic-Inspired at best since you can totally see similarities but it tries to live beyond it with it's own style.
I guess it depends on what you mean by "clone". The basic movement controls and level design style (big huge stages with loop-de-loop and momentum building runs) are more or less lifted straight from Sonic, though they're all heavily tweaked (for the better, IMO). The new moves and mechanics though are pretty original, and do a good job of improving the Sonic formula, and making the game stand on its own merits with a unique flavor from Sonic.
I think a good comparison would be (classic) Castlevania vs Ninja Gaiden. NJ basically took CV's play style and mechanics (sub weapons, commitment to jumping and attacking, freaking HUD completely ripped off, enemies that knock you back into pits etc.) and put its own spin and refinements on them.
Castlevania is made unique by its need to commit to attacks and jumps (both of which lock you in one place/direction and can easily be punished by enemies if you misread the situation). Ninja Gaiden gives you the ability to alter jump trajectory with the caveat that you can't change jump
direction but can only change momentum a bit to slightly alter where you'll land. This means that jumps are still a heavy, heavy commitment (as in CV) and won't let you get off scott free if you completely misread the situation, but you have greater leniency now to make the game less punishing. For attacking, Ninja Gaiden made player slashes near instantaneous (as opposed to CV's deliberate and punishable start up and recovery time on whips), but also made the sword in NJ much closer range, forcing you to get closer and be more aggressive with enemies which is just as dangerous a commitment (also forces you to move around more). Lastly they made the basic move speed a
lot faster. The end result of all these tweaks is that NJ has a similar focus on commitment that CV does but is also a more movement focused and aggressive game with greater leniency.
Ninja Gaiden is arguably a "clone" of CV on a really basic, mechanical level (
and dat hud!!!), but fine tuning and tweaking make it its own unique experience that's equally valuable. Similarly Freedom Planet is a unique spin on an older concept that puts its own spin
ball on things and easily stands apart from its inspiration.