I don’t live in the EU, but that makes sense to me, though I’d expect it to be something like 11.7 (9 + 30%) instead of 11.8 (27% would be somewhat less)*. 
I do have some experience in this area, in case it helps with some explanation. Although I am not a law or tax professional, I’ve worked with them on these sorts of compliance issues in the past, rolling out changes exactly like this one with accompanying documentation and info for both customers and customer support folks – it’s definitely confusing for all involved!
While it varies depending on locality (of both company and customers!), in many cases small companies are not responsible for collecting and remitting sales tax at all. I suspect that may be where Tofugu started. Often, at a certain point though, it becomes required, regardless of where the company is based, because the revenue in (a) certain legal jurisdiction(s) meets or exceeds a set threshold.
As a concrete example, a company I worked for previously had exactly this happen in, of all places, Japan, and received a tersely-worded letter from the NTA (Japanese National Tax Agency), which came as a bit of a surprise. We scrambled to be compliant before the deadline, as revenue had steadily increased from Japanese customers over time to the point where it became our responsibility (even as a North American company) to collect consumption tax from them on behalf of the government in order to pay it ourselves directly for each sale within Japan (meaning using a credit card with a billing address in Japan). Later on Japan changed consumption tax (there’s an 8%/10% split now) which had to be taken into account as well, though that was simpler technically and more a matter for communication.
The other case where this sort of thing (tax not assessed previously now required) comes up a lot is simply that the law changes, though it was not incumbent on a merchant to collect and remit tax at all under previous law.
Especially on the internet, this is confusing and complicated. The company has to do this, but unless it is known that the entire customer base is in one particular jurisdiction, you can’t very well say, “The price is 11.08 inclusive of tax” on the website because it simply won’t be true in many cases (in the US, sales tax varies by state, and cities also often have their own as well).
Anyway, the “marketing” pages for WaniKani and its billing system likely aren’t setup to be at all dynamic for this sort of thing since it’s a relatively new development – it simply wasn’t relevant previously. For our part, when I was involved on working on this problem for another company, it was determined that marketing pages would just be changed to mention that tax may be assessed if necessary, and then internally the billing system would reflect it during checkout for each individual customer. What Tofugu does may vary based on their legal counsel, but I suspect they’re working on it – though existing customers likely wouldn’t see any changes to the checkout system that reflect the new tax situation, even if they go that route. There’s just not really a perfect solution no matter what, but especially for existing customers who in spite of needing to be charged aren’t necessarily going to see any attempts at communication since they’re probably not going through checkout on the website again (or, like me, not going to the website at all because they’re using a native app to access the service). 
*But there are taxes imposed by country, municipality, other divisions, broader organizations like the EU, and in many cases more than one need to be taken into consideration, and vary greatly across even small distances… This is why I haven’t gone into that career myself. 
