Splitting = and := is intentional, but not for the reason you stated. We could have used := for all assignments from the beginning:

    is_logged_in := True

or

    if is_logged_in := True:

I agree that this would require blurring the statement/expression distinction. You can still do that in a weird way, by disguising your assignment as an expression. This is valid:

    (is_logged_in := True)

The reason it was done that way because := was an afterthought, and making it the assignment operator would have introduced a breaking incompatibility. That lead to having 3 different symbols for 2 use cases (assignment and comparison).