Most organizations actively try to support internal mobility (admittedly with varied success). Companies invest resources in hiring, training, and keeping you. Your manager, as a representative of the company, is wise to amicably transfer an unsatisfied employee to another part of the organization. And as a human being, they probably want to see you engaged and happy at work, even if it means losing you to another team. Plus in this example, OP was a high performer and had already shown initiative, putting in legwork to upskill themselves for the new role. Demonstrated value/performance and initiative are always helpful when asking for something of your employer.
Don't be dissuaded by cynical examples to the contrary - those who told their manager about other interests and suddenly and inexplicably got fired. Remember there are two sides to every story - for example, maybe they were underperforming and disengaged for a long time, and then randomly raised the question after their manager reached a breaking point? They won't share that in their post. Also remember availability bias - just because we see these people complaining on forums or "know a guy" doesn't mean it happens frequently enough in real life to be seriously concerned.
In the end you'd be surprised what you accomplish by simply talking to your manager early and often, having general conversations about potential aspirations and leaving the door open for them to say "How can I help?"