However, we are talking about an arbitrary decompressor here. The decompressor WASM is sandboxed from the outside world and it can't wreak havoc on your system, true, but nothing stops it from producing a malicious uncompressed file from a known good compressed file.
If the compressed file is malicious, it doesn't matter whether it's malicious because it originated from a malicious uncompressed file, or is malicious because it originated from a benign uncompressed file and the transformation into a compressed file introduces the malicious parts due to the bundled custom decompressor.
