Bankroll management is a crucial part of gambling and with every strategy you use they must be in line with bankroll management...what I mean is very simple, if you are using a strategy like the martingale for example this doesn't really work well with bankroll management, as a matter of fact it's one of the ways to lose your bankroll faster....if your strategies are not in sync with managing your bankroll then it's not worth trying out
So, in a short comment, bankroll is actually a strategy but it is not like the other strategies of gambling which players are adopting for the purpose of winning. Like the Martingale strategy that you mentioned, people tend to use it because they want to win but no they don't respect bankroll management since it doesn't seem like a strategy that can make them win. Well, I just see the bankroll management as still a strategy.
Honestly am just amazed at how many people including the OP is saying that bankroll management is not a strategy. In the real context, what is even meant by ''a strategy''? lay down plans for future achievements, isn't it?. Now when we are talking about bankroll management which what will sustain our gambling budget in the long term. Honestly, bankroll management is a strategy that is set to manage our money that is set aside for gambling, if it's not a strategy then we can just use up our bankroll within ''a blink of an eye''. So, for me, bankroll management is a strategy.