Anything and everything about "Spiritualism" is anti-scientific, because "Spiritual" things are outside of nature, (eg, Super-natural) and cannot be examined by the scientific method. (the cornerstone of the scientific method is repeatable results; specifically, results that are repeatable by OTHERS following a stated process, allowing refinement of true knowledge about the world through experiment. EG, OBJECTIVE study. The " imagination, inspiration and intuition" that they proclaim to use, by their nature, CANNOT be objective. These are inherently SUBJECTIVE things. As such, "Science!!" cannot be applied.)
So... Once that door is open, there's no telling how deep they will go down the rabbit hole. BUT-- this is true of ANY religion that asserts the existence of a supernatural world.
That's subtly but critically wrong.
Methodological naturalism does not technically assert that only natural causes exist, as philosophical naturalism itself does; it assumes arguendo that all causes are natural, but assumptions differ operationally from assertions. Spiritualist claims are perfectly testable where they intersect with the observable universe, which they often do during the sales pitch. We can test crystals for magic healing, see if mediums can recall factual information about deceased persons, test the clairvoyant with Zener cards, quantify the power of prayer, put alternative medicine through controlled trials and so forth. The critical thing is this: we do not need to believe the supernatural exists in order to investigate the allegedly supernatural. Moreover, this still allows for putatively supernatural results. If it works at all predictably, we can make predictions about it, and then it's not supernatural anymore. It's just natural. Subjective things can be tested by this method too, most familiarly through double-blinding and certain statistical methods to normalize bias. It is even possible to test for the physical relevance, if not the existence, of various spirit realms and so forth supposedly in communication with the physical world, just by varying the effervescent aetheric flux or whatever woo they're claiming and seeing what changes.
It is true that the existence of things that have never and will never interact in any way with the observable universe is not testable, but that does raise the obvious question of how the spiritualists learned about them and claims of that sort are definitionally irrelevant anyway and make a poor basis for a religion.
Just because it's not worth testing nonsense doesn't mean we can't, you know.
(Oh, and we're not producing 'true knowledge'. We're making predictions with mechanistic implications. It's an important distinction when looking at how theories change.)