That's it right there. That ground wire from the mains cord needs a dedicated bolt. Anchoring other grounds to them is not technically against code, but it's not a great practice for long term durability. Transformers heat up and cool down and they have a lot of mass, both of which contribute to those nuts trying to work loose over time. Later CBS era amps that use them often have loose grounds from the fasteners loosening and corrosion under the wires where they meet the chassis. There's a couple of reasons code is written this way. One is a severe impact can rip a transformer free of a chassis. We're talking UPS Gorilla throwing packages level of impact here. The other worry is if someone replaces the transformer, there's a chance (Murphy's Law) that the safety ground may be forgotten when replacing the nuts. That's why a dedicated fastener is used. That ground is what will save your life if there's a fault that makes the chassis live.