This is good advice. Onions tend to take their time, meanwhile the garlic with them burns and loses flavour, just waiting until onion is ready to go out, but onion is still getting ready. Always getting ready. Onion needs to put its face on. Onion doesn’t care that garlic is aromatic and ready and has been patiently waiting for it to start even softening up. Onion is selfish. Garlic shouldn’t even bother getting pressed until onion is ready.