The form in which the Sefirot appear is spiritual, yet even this is not part of their intrinsic essence but only the form in which they appear.
Even the spiritual forms the prophets see are forms of appearance, not intrinsic essence. We cannot know the Sefirot in essence — that would require knowing Godliness in essence.
There is a subtle misreading Op. 7 and Op. 8 leave open. A reader might think: the bodily images are not the Sefirot — fair enough — but the spiritual form behind them must be the real thing. Op. 9 closes that misreading. Even the spiritual form is a form of appearance. The Sefirah's intrinsic essence is, like Eyn Sof's Essence (Op. 1), beyond what we can know.
This is the same epistemic restriction Op. 1 set, applied at a new level. Op. 1 said we never speak of God's Essence; only of His Will. Op. 9 says we never know the Sefirah's intrinsic essence; only the forms in which it appears. The two restrictions are connected: if the Sefirot are attributes of His Will (Op. 6), then knowing them in essence would be knowing His Will in essence, which would be tantamount to knowing God in essence. The restriction at Op. 1 cascades down to Op. 9.
This is not pious hedging. It is a methodological discipline that lets the rest of Kabbalah operate without overreaching. We can investigate how the Sefirot appear, how they couple, how they govern. We cannot investigate what they are in themselves. Every operational claim in the rest of the book is at the appearance level; every claim about intrinsic essence is held back, as Op. 1 holds back claims about His Essence.
Op. 25 will sharpen this in cosmogonic terms: the Sefirot are Godliness in essence (bound up with Eyn Sof), but their visibility is willful — what we see is the radiance of His splendor, not His intrinsic essence. And Op. 85 will introduce radla — the Unknown Head — the structurally unspecifiable level above which inquiry cannot reach. Op. 9 plants the discipline; the rest of the book honours it throughout.