The article you linked is about how Steam geo-locks activation keys to *prevent* UK customers from purchasing games outside the UK to avoid paying UK VAT rate - that's the exact opposite of what is happening here.
I'll say it again, Steam are *definitely* charging 20% VAT in the UK. https://partner.steamgames.com/doc/finance/taxfaq
In fact, here's my invoice from A Short Hike.
From what I can tell the developer has either set their Itch price incorrectly, or their Steam price incorrectly. And I have seen this happen several times. The mix up means Steam ends up charging 20% less than Itch. This needs to be addressed, it makes Itch uncompetitive.