It is not that simple. Following my naturalisation ceremony, I had to subsequently prove that I had indeed renounced my British citizenship. Seeing as I had to submit and pay for the renunciation process to trigger the naturalisation ceremony, there was no way around doing it. I was also required to furnish the “registered” (ie. formally approved) renunciation of my British citizenship bearing the Home Office stamp and date with MA35 to conclude the process and for my file to be closed.
Article 20 of the Austrian Citizenship Act of 1985 clarifies that renunciation must be done within 2 years of receiving Austrian citizenship. (Information checked on 05.01.2019).