You may find this helpful: What is McTaggart's vicious circle problem with the A series?
In the comments Conifold says: "McTaggart was an objective idealist, all subject bound constructs were "unreal" to him."
So McTaggart wants a scientific alternative to the A series that does not involve an observer who fixes the present moment by existing, and the B series is just that: no past, present and future, just one event before, simultaneous with or after another.
This is fine for an abstract scientific timeline, but from a phenomenological perspective the observer is not omittable. For example from the IEP on the phenomenological reduction:
scientists take the world to be their axioms; and it is this axiomatic
status that Husserl throws into question when he shows that the
results of scientific investigation are a function of both the
architectonics of scientific hypotheses and the psychological coloring
of the investigating scientist. For this reason, Husserl says that if
we are ever to be able to access the pure world so that it can act as
a proper foundation, we must strip away both of these qualifications
and return to the “things themselves” [die Sache selbst]. That is, we
must return to the world as it is before it is contaminated by either
the categories of scientific inquiry or the psychological assumptions
of the scientist.