As a character yes Xehanort's heartless was inside of Riku which is what Nomura is referring to there and it's all that matters to Riku because it means Ansem is very much alive in him. But the actual heart of Xehanort was not, Ansem effectively shifted which heart he existed in. The series ultimania that gives us a comprehensive explanation on Xehanort and his timeline even put and to this age old debate because it directly states that Ansem ceased to exist as a heartless post KH1 and instead existed as Riku's darkness.Nomura makes it pretty clear that Xehanort's Heartless actually did still reside in Riku.
Like it's not just a metaphor/symbolic thing. Riku really did hold the remnant of Xehanort's Heartless.
It's an important distinction for two big reasons. The first being it makes him a separate being from Xehanort whose existence post-KH1 had no bearing on Xehanort's revival and also why the games and even Riku himself always make the distinction that this is "Ansem's shadow" instead of being the actual heartless AnsemSoD. The second gets into DDD spoilers so I'll tag it....
By the shadow's own admission Riku can't get rid of him as long as Riku has darkness in his heart because the shadow literally is a part of Riku's darkness, a part Riku. So no matter how many times it is destroyed he will return which is why he smiles as he explodes because he knows it's only temporary. And return the shadow did but Riku never again lost control of him...though he did lose the ability to lock away when he went all out for Roxas. The encoder explosion pushed the darkness back inside Riku and he returned to keeping the shadow locked away.
By Nomura's own admission the story of Riku pre-DDD had been learning to lock away the power while the story of DDD is Riku growing, accepting that power, and making it his own.
Nomura: I heard from many fans that they wanted to play as Sora again soon, and I felt I wanted to answer them. However, the true protagonist in plot for this title isn’t Sora, it’s Riku. The story depicted in this title is of Riku’s growth as he changes stance and makes the darkness he had been locking inside himself up until now into his own power. This is symbolised by Riku being chosen as a Keyblade Master at the end.
And this is what that whole dialogue between AnsemSoD and Riku is at the end of DDD.
"After all your efforts to control the darkness it's a shame you locked away that power"= You learned to use my power then locked me away, use it again so I can have a chance to take control of you.
"Set the power free and you can rescue your friend"= Use my power and we'll work together to save Sora....mostly by us replacing him as the 13th vessel.
"Even after locking you away here you are haunting me again, I get it now there is no point in holding the darkness back"=Even if I lock you up that doesn't make it impossible for you to escape, us talking now proves that, and so I understand keeping you locked up is pointless because it doesn't solve the issue.
"At last you see clearly"=Finally you understand your attempts to hold me back are futile now set me free.
"I know the way consume the darkness return it to light"=I'm going to do to you what you tried to do to me, I'm going to consume you and make your power my own and by doing so you'll cease to be.
"Ansem you are part of my heart part of the light!"=Give up you're a part of me not the other way around and I will make you accept that!
"Ever the fool boy and forever a pawn of the darkness"=You're wrong you're just my pawn, my vessel, and that is how you will be forever there is no escaping it.
There are were two Ansems in DDD (a parallel to CoM) the one who had taken over Riku and was grabbed by Young Xehanort to be part of the Organization and who spoke to us in La Cite Des Cloches. And the one shadow who tried to make one last desperate grab for Riku when Riku was sucked into the abyss. He even mimics what he did in CoM, starting out as just a voice speaking directly to his heart and not appearing until Riku was sucked into the darkness. The series ultimania even refers to them as one being a time traveler and the other being his shadow.