Thanks guys, I also confirmed that it's Type 4 by looking at one of the unused cables - it's marked on the connector. I did see that a month ago when I replaced the GPU, but totally forgot about it.
Honestly that's the one thing that had me concerned. Seeing it made me think that there are HXs that are not compatible, and although that might mean that HX Silver/Gold is not supported, it would've been much clearer to just say HX (Platinum) in the list of supported PSUs.