It's not merely a matter of "glycol," it's a matter of which glycol. It appears easier to gain access to top secret CIA documents than to find out what's actually in Tamiya paint—and Gunze is worse—at least unless you read Japanese.
Golden retarder contains propylene glycol, but obviously contains other materials, as well, from the MSDS (which provide information only on "hazardous" components and are not required to list "proprietary" ingredients). I know that some acrylic polymers are incompatible with various forms of glycol, but not with others. This would seem to indicate that propylene glycol is universally compatible. However, I also know that retarders are only marginally compatible with some of the acrylic polymers used in hobby paints. (Yes, I actually tested this.)
Glycols generally reduce evaporation rates in either alcohol or aqueous systems. However, some also interfere with acrylic polymerization, making them ideal for acrylic systems. There are also other chemicals which interfere with that polymerization, and I suspect they are present in some acrylic retarders, since the result of adding too much retarder (or adding it directly to the paint) is rapid flocculation.
It therefore seems a reasonable assumption that proprietary thinners that contain a retarder contain propylene glycol (possibly with other chemicals), or they contain a different glycol that is compatible with the acrylic polymers in that particular brand.
Please remember: too much retarder=ruined paint.
Phil, how did you find out that Tamiya and Gunze contain glycol? I've been trying to dig up that information for years!