It barely existed as a female name until the 80s/90s. Traditionally, it is very much a male name. If you look through all the famous Sidneys and Sydneys on wikipedia, you might not find even one woman.
I think you're misunderstanding what's being shown in the plot.
If you look at the actual data, Sydney barely existed as a name for either gender for a long time. Then it became a very popular female name (top 25), while still barely existing as a male one.
To illustrate: in 1960 there were 128 female Sydneys and 52 male. In 2000, there were over 10k female Sydneys and 126 male.
After the 80s/90s though it seems to clearly be a female name. For someone born in 2023 named Sydney it's 20x more likely that they are female. If you search just "name Sydney" in wolfram alpha the result even says "Assuming Sydney (female)"
