Yeah. I believe the theory. GameFreak usually does these type of mysterious things, so...
The body features are somewhat alike, and the characteristics are pretty similar too. The name sort of gives it all away on what GameFreak might've did, since Gengar's name is very similar Dopplegänger and Genganger. The definitions of Dopplegänger and Genganger fit very well to the theory- Dopplegänger and Genganger both generally define a living thing's ghostly double, and Gengar is believed to be Cleffable's shadow/ghostly form.
Although, be don't know what GameFreak has done, there is great evidence supporting this theory. It may be a coincidence, but we never know...