Wasm doesn't have its own GC, but just like Go binaries for other compilation targets include a runtime with GC, the Wasm binary includes that as well I think. That's regarding Go in general.
The post is using TinyGo, which explicitly mentions it uses a GC and doesn't exclude the Wasm target (which otherwise is sometimes mentioned if there's something it does differently): https://tinygo.org/lang-support/#garbage-collection
TinyGo's wasm output absolutely does include it's own garbage collector, though it's only been in the last few point releases. eg maybe um.. 6-9 months or so (rough guess)
It can be turned off, but by default it's included.
From rough memory, it's based upon or uses the same principles as the MicroPython one. Akye (primary TinyGo author) was experienced with MicroPython before starting the TinyGo project.
Disclaimer - I've made very minor contributions to the TinyGo project, but haven't really touched it in a few months.