I think you misunderstood a diagram; grams of Al per square centimeter is a standardized unit of shielding in the literature, not an engineering choice.
You should realize that there's no simple way to convert from aluminum to polyethylene: it's not as simple as "PE is X times better." Due to secondaries aluminum stops improving after 30 g/cm^2,[1] hence why their chart (bottom line) levels out. PE doesn't have that problem, and lower dose rates are achievable versus what the author states.
TL;DR regardless of if you call it a "standard" or not, the author is still using bad math to overstate the actual risk.
[1] Figure 3, solid black line https://ntrs.nasa.gov/api/citations/20170005580/downloads/20...
You can read the author's other work, that goes into great detail about different types of shielding, if you want to gain confidence in his math. The upshot is you need many meters of polyethylene to effectively shield the heavy ion component of GCR, which is what the fuss is about.
But the point of that particular diagram is not shielding, but to illustrate the 2-3x uncertainty in estimates of tumor risk based on our poor understanding of high-Z ion exposure.