That doesn't matter though. Since it's not representative of the real shielding, it's an inapplicable standard to use for this problem.

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...