The Kingdom Hearts in KH1 was as "real" as the one in KH2. "Real" is probably the wrong way to describe it.
The KH in KH1 was an artifical KH created from the hearts of the worlds that Ansem SoD plunged into the darkness by sending his Heartless Army to destroy them.
The KH in KH2 was also an artifical KH, but created from countless human hearts captured by Xemnas and Organisation XIII after Roxas (and later Sora) had freed them by destroying the Heartless that held those hearts.
Instead of being truly set free and being able to revive the humans that were once lost, these hearts were held prisoner by Org XIII for their KH (and that's why Sora and co. also were completely in the right to destroy the Organisation and why Xion asked Roxas to free all the hearts they gathered, because she realized how disgusting and depraved the Organisation really was. One should not forget that each heart stands for a consciousness, an existence).
So both of these KH's are "artificial" because they were created by forcibly merging together hearts, as in hearts of worlds in KH1 by Ansem SoD, and human hearts in KH2 by Xemnas.
They are both also "real" KH's and hold massive power, but in comparison still inferior to the one true KH that was lost in the Keyblade War.
It's the same as with the X-blade forged in Birth by Sleep. It was a real X-blade with tremendous power, but only a 2/20-part of the perfect weapon and due to this and Ventus resisting (making one of the two parts gathered together effectively diminishing the stability of the already fragile weapon even more) it was highly unstable.
Xehanort says this as well in his reports:
Xehanort Report 7 said:
Unlike the Kingdoms brought about forcibly and artificially through the collection of hearts, THIS Kingdom Hearts is a perfect and complete union of ALL the worlds' hearts.
It is the way
how the union is archieved that matters.
The KH in KH1 was made out of world's hearts, yes, but 1st not of
all world's hearts (only those which had fallen to darkness) and 2nd not accessed through the X-blade.
The door to darkness itself also isn't the KH of KH1. The DtD is only a pathway into the realm of darkness where the KH of KH1 resided.
It is also said in Dream Drop Distance by
both Yen Sid and Master Xehanort that the true KH is "unseen" even during the era of "fairy tales" when the world was one, meaning that the true KH has no visible form.
