Not true.
Martin Luther translated a large portion of the Bible into German using the court German of the House of Wettin.
The Electors of Saxony - the major political power in the Saxony/Thuringia region - had been using that written dialect of Saxon German for more than a century for official correspondence, record keeping, etc.
If you lived in Wittenberg and owned a business, or were a guild craftsman, or engaged in trade at a high level, you could likely read this Kanzlerdeutsch, and you could likely read simple Church Latin as well.
Part of the Luther legend is the implication that he somehow invented the use of written German.
What he did was take one of the better-known written dialects of the language and make it the most popular and widely-used written form of German.
Academic Latin - like that of Thesis 20: Igitur papa per remissionem plenariam omnium penarum non simpliciter omnium intelligit, sed a seipso tantummodo impositarum. - would be completely unintelligible even to the average literate citizen of Wittenberg, let alone the unlettered mass of citizens.
The 95 theses were written for a handful of advanced students of theology.
In contrast, Luther wrote his German works, including his translation of the Bible, for a different audience.
Those were written for the nobility, the petty nobility, the wealthy tradesmen and guildsmen and mid-level clergy.
His core constituency.
It's been 45 years or so since I last tried to read Latin, but I can make out Thesis #20. Whether the average Wittenberg parishoner could or not, I don't know, but Pope Leo chose to communicate with the Saxons in Latin, rather than German, when he delivered the Exsurge Domine and ordered that it be nailed to church doors. That Luther would choose then same method of communication as the Pope, when seeking to communicate with the same people, could have been mere coincidence, but maybe not.