Question: why odeadvisor gives this ode type _dAlembert?

dAlembert ode has the form

https://fr.maplesoft.com/support/help/Maple/view.aspx?path=odeadvisor/dAlembert

Which also agrees with textbooks and Wikipedia

https://en.wikipedia.org/wiki/D%27Alembert%27s_equation

So the ODE should have   x  in there (the independent variable).  

Then why  in the following, when I give Maple ode of form   y=f(p)+g(p)  it says it is d'Alembert? notice that there is no x in there

restart;
ode:=y(x)=ln(cos(diff(y(x),x)))+diff(y(x),x)*tan(diff(y(x),x))

DEtools:-odeadvisor(ode)

I am not saying Maple is wrong, as I am not sure, but I am just trying to understand what is going on. All references I've seen so far say x should be there.  Otherwise, how did Maple determine it is dAlembert if not using the orginal form to check against?

The strange thing is calling dsolve on the ode, with no option gives different answer if calling dsolve telling it is is dAlembert:

dsolve(ode,[dAlembert]);

but

dsolve(ode)

Why it did not give the same result?