UK EASA licensed TREs could sign the license from any EASA state before the end of last year. However, one had to bring the national forms for them to fill out and comply with the relevant rules around them, but that is certainly not rocket science. So yes, one could SOLI out before the transition period was over, but of course, that took a minimum amount of work.
And no, life is not fair, it never was and never will be, most people learn that very early on. Kindergarden is a normal place where that starts to sink in. In this case it was of course aided by the fact that anything to do with the ECJ, even if it was very far removed, was completely impossible to accept for Lord Snowflake, while the EU is a very much rule based organization (which does not mean that they can't change the rules very fast if needed) and therefore follows its own rules. The UK wanted to be a third country, Ok, lets start from the status of a third country without any relation to the EU and negotiate upwards from that. Aviation and especially licensing was not very important to either side.
As to the requirement by Discover: not unusual anywhere in the EU, but in this case probably even excusable, after all German is by far the language with the most native speakers in the EU both before and after brexit. Close to a 100 million people in Europe do speak some form of german natively, in quite a few countries, and some do learn it in school, as most of Europe teaches at least two foreign languages in school (one of them usually being english).