I agree that the writes are synchronized with the reads in my example. But the reads are totally unsynchronized with respect to other reads, in every sense. That is what thread-compatibility guarantees you: concurrent reads without synchronization. A thread-unsafe type doesn't allow even that much.
"Read" here is shorthand for "call a const method". You can write const methods that are not safe to call concurrently: for example, they could use "mutable" or "const cast" to perform mutation, or they could access a non-const pointer to shared state. If a type had a const method like this, the type would no longer be considered thread-compatible.
