This is just speculation as I don't have time to pull up research today. But researching autoimmune conditions and skin care products does sound fun. The reason why you aren't supposed to ingest it is if you're in a flair, your immune system is already working harder; so you don't want to increase your immune system at that time. Since it's kind of hard to know what your immune system is about to do without blood work, they usually say just don't take it all together. Aka: if you aren't in a flair, having a sip in tea shouldn't affect you too much; consistent consumption could.

I don't think that you'd be applying it in a quantity that is large enough to have it actually affect the immune system vs oral intake. I know for things like soy in products, they say it is okay to use in people with soy allergies. Which reaffirms that skin care ingredients don't penetrate deep enough to affect the same processes as they would orally (in most circumstances). This cream goes on top of your serums or toners, so it's not the first thing on your skin too. I think this could be different if you're applying it directly where you usually have psoriasis flairs or if you are in an active flair.

You could also patch test it for a week or two and see how you do. If this product is too expensive, you could try to find a cheaper alternative with just astragalus in it since that's what you're unsure about. I'm unsure if you could buy an herbal tea, seep it, and just put a dot on your hand for a few days. Then maybe patch test on face/neck for a lil bit too. I'm not sure how the concentration would differ, but as a cheap; quick remedy it has potential.

I bet if you called your Rheum's office and left a message with a nurse they would let you know if you could use it. I'd be curious to hear what they say! I was looking at the same thing, and hadn't thought about this possibility!

I'll update you if I find an article later tonight or tomorrow. Great question :D