In German, the direct equivalent to aikidoist would be something like Aikidoer (in analogy to Fechter, Ringer etc.).
But, in the early sixties of the last century, when aikido came to Germany, many early aikidoists were equally judoists and members of the DJB (Deutscher Judo Bund); so they probably took advantage of the superficial analogy between "Aikidoka" and "Judoka", this in common use for a judo-player then and now.
Like in many other countries, the term "Aikidoka" is now widely used and apparently has evolved into a kind of appropriate one-word description, singular or plural, without (anybody ever caring about) its deeper meanings in Japanese.
As I said earlier, I had never heard the term `Aikidoka` from the 1950s untill maybe the 1980s - In 55 years of m/a I had never heard the term `Judoist` - as I read the above post I wondered why ? - In the early days it was always a Judoka and an Aikidoist. It was the same with the title Shihan, I am not sure when I first heard that used - maybe the 80s.
Co-author of `Positive Aikido`