That’s right, but perhaps I can give a stab at a more technical explanation.
An RMBS is a portfolio of mortgages financed by multiple bonds, with an order of subordination between them. When a loss occurs on a mortgage, it is allocated to the most junior bond first, until it is fully written down, then the next junior bond, etc. So if you own the most senior bond (often rated AAA), you are really exposed to high default rates and high correlation. Ie if the portfolio of mortgages is diversified (low correlation) some mortages will default but not all at the same time, and there will always be enough mortgages that do not default to avoid a loss on the most senior bonds.
The big fuck up in the financial crisis is that the correlation was misestimated. There had never been a large, US wide, real estate crisis since 1929 and therefore everyone assumed a US wide portfolio was well diversified. That turned out to be wrong in 2006-2007, and delinquencies (ie balances of loans not paying interest or principal) shot up. This led to losses (not 100% of a defaulted loan, the house still had some residual value) that ultimately hit the AAA bonds.
A CDO is the same structure but using bonds in the portfolio instead of residential mortgages. But when people refer to large losses on CDOs in the financial crisis they usually refer to CDO of ABS, which basically uses junior RMBS bonds in the portfolio, so effectively you leverage the mortgages twice, first through an RMBS, and then another time through a CDO.
In those you are even more exposed to correlation. The portfolio is made up of junior RMBS bonds from multiple RMBS transactions originated by multiple subprime lenders across the country, so these structures were playing on not all of these RMBS going bad simultaneously, which is exactly what happened.
So the TL DR is that these portfolio of subprime mortgages and RMBS bonds were of a similar credit quality, but correlation is what got really badly mispriced, they were assumed to be diversified but what we got is a large US wide real estate crisis that hit everybody,