WebGPU just shipped today in Chrome, there are no people reporting that the demo doesn't work with their days-old browser, so it doesn't use WebGPU.
While it's possible, without WebGPU it's really tedious to run NN in the browser.
Also, the model is implemented in PyTorch and wasn't converted to other model format for other runtime. While technically you can compile CPython and PyTorch to WASM and run the duo in browser, there are definitely no GPU access.
Given that they explicitly mentioned the decoder was converted to ONNX, it's obvious this isn't done for the encoder and they really mean PyTorch, running with Python, on a server.
Okay, so you browser can't run the encoder, yet the web demo works, it's quite obvious on which server the encoder run.