I am sorry, but I don't believe that Toyota switched coolants to pre-diluted pink due to them switching to a all-aluminum engines starting with 2004. Based on what I have read, the main difference of Toyota Pink is that it comes pre-diluted with distilled water, which on average makes it last longer due to the fact that most consumers and most Toyota technicians were not diluting the Red with distilled water.My 04 came with pink, I'm the first one to change it. Pink is for all aluminum systems, such as mine. When you mention they both have "proprietary corrosion inhibitors", you are assuming they are made up of the same chemicals.
Both Toyota Red and Pink are Hybrid Organic Acid Technology (HOAT) coolants. Although there may be some minor differences between the two, the differences are relatively small and are compatible if diluted properly. Many Toyota engines (if not all) were all-aluminium or substantially aluminum for a long time prior to introduction of pink coolant. I realize that there are some who disagree with me.
Here is something I found from Toyota:
"ENGINE COOLANT COLOR CHANGE
A new pink colored LLC is being phased into production for all 2002 Japan Built Toyota models. The color change does not affect the maintenance procedures or intervals established for the red coolant. The new pink coolant is 100% compatible with the current red coolant and red coolant can be used to replace, top off, or mix in with the pink coolant with no adverse effects."