It's his english fans who called him spoken spanish fluently, but kinda different with spanish people who rated him as average - good.
However, he's still very good consider he's pretty new in Spain. He's good when talking spanish with his own unique accent, but you can compare him to the Modric or Kroos, which are speaking spanish fluently.
This is not fundamental, I do not think that Alexander Arnold has deliberately learned Spanish just to move to Real Madrid from before, maybe it happened unintentionally and I think language barriers are not a barrier for all players to go to any team now and for him Alexander Arnold can already speak Spanish and that will make chemistry stronger with other Real Madrid players regardless of whether or not Alexander Arnold speaks Spanish fluently.
Even last season when Alexander Arnold was not in his best form in several matches, Liverpool fans had mocked him and said that he was no longer comfortable with Liverpool and wanted to go to Real Madrid, I think that is very unreasonable, everything is only linked by irresponsible fans or media.
It's just to clarify if called him spoken spanish fluently is just wrong. However, still appreciate him considering he left in March, then able to speak spanish that good even not as fluent as people think.
I also tell you i've heard a rumor he has been learning spanish a year ago, and it proves how that rumor is true.