The report doesn't read like something involving 500 rem and potential death in 3 weeks. It says "Non Emergency". Can you link to this converter? It seems to be a rather key step that got handwaved. Wiki says [0] there isn't a standard on what a "count" counts.
This website [0] gives the same numbers (300 CPM -> 500 REM). Seems like a candidate for what was used at the very least, and nothing else obviously appeared to claim a similar CPM -> REM conversion capability.
Assuming this website was used, it looks like it does a naive multiplication by 5/3, which seems... simplistic? The rest of the page doesn't exactly fill me with confidence either. No indication of how the conversion factor was derived and there's a bunch of links to other CPM -> <radiation-related unit> calculators. On top of that, the landing page for the root domain boasts about AI capabilities and their AI page prominently features "Elevate Your Content Creation" and "Generate high-quality AI content with ease!"
I'd love to know how they got to 5/3. It also offers CPM to half-life conversion which has to be at least poorly labelled. That would imply that if two piles of different radioactive substances emit the same amount of radiation (obviously different masses in each pile) they have the same half life. That isn't the case, half life depends on what the substance is which is radiating, not the measured amount of radiation emitted.
Given their CPM to half-life conversion amounts to dividing by 60 (not to mention the nonsensical units), I'm not sure I'd place much faith in the website at all.
[0] https://en.wikipedia.org/wiki/Counts_per_minute