Yes I agree with you that bankroll management is not a gambling strategy. The former is the process or act of setting limits on how much you will spend on gambling over a period of time. Another name for it could be budgeting. Strategies are plans or patterns adopted to win games. Different gamblers have different bankrolls based on their financial strength but they can use the same strategy.
You are right that gambling strategies are what gamblers do while gamblingro win games. But what is happening to most gamblers? They are losing. Gambling plans and strategies goes beyond that, it even started from the bankroll or budget that you use or set aside for gambling. If you do not believe me, you can browse about it online.