can I use DC battery cable rated at 600A to connect a 63A main AC breaker to a 40A RCCB in my distribution panel?
Technically, you could use it. You won't burn your house if done properly. And the insulation should be enough or it's really very cheap low quality wire. But... it's really unprofessional. Sorry.

When you do this kind of job on electrical installations which will last forever, better do it properly and lay some extra bucks for the satisfaction of doing something beautifully.

The biggest problem with battery wires is that they are multi-stranded. You need single strand for main electrical installations. This is very important. In some countries it's allowed to use multi strand wires but a termination must be added to keep all the strands together before inserting them inside the circuit breaker. Yet I don't know the regulation in your country. In any case it looks more professional to use single stranded wires because that's what every professional do.

Somebody talked about fire insurance. If your building burns and the insurance expert finds your battery wires, they will conclude that the installation was not done by a certified electrician. If they find a good cable, they won't think that you did it yourself.
It's also a good investment, the day you or its owner want to sell or lent it.

For very large gauge, only multi-strand exist because it's not convenient to make them single strand. But these strands are very thick in comparison to true multi strand wires.

Multi strand wires must be used only on constantly moving elements.