That's correct: in the 1967 redesign the Valiant 200 was dropped as a separate model and the name became an optional "decor group" for the 100. Attached is an extract of the 1967 Plymouth Valiant Car and Equipment Prices card. I haven't seen the cards but I assume this was carried over in 1968 and 1969. In 1970 the 100 and Signet model names were dropped altogether.
To put the $72 cost of the 200 decor option in perspective, the price difference between 100 and Signet models was $145 in 1967. And $145 represented between 6.5% and 7% of the base MSRP of a Valiant 100.