DevOps started as a culture. It worked beautifully at small scale, where every engineer could hold the entire system in their head. At larger scale, it produced 'you build it, you run it' as a slogan but also a tax — engineers spending more time on infrastructure than features.
Platform engineering is what happens when you build paved roads. The platform team treats internal engineers as customers, ships an internal product, and measures itself on developer experience metrics.
Done well, the platform raises the floor without lowering the ceiling. Done poorly, it becomes a gatekeeper. The difference is product mindset and continuous user research with internal teams.