I highly doubt that was the case. At the beginning, way before Tamiya entered the smaller scale car model market, the competing scales were 1/28, 1/24, 1/21 and 1/20 among the Japanese kit makers. 1/28 and 1/21 soon fell out of favour, leaving the market divided between 1/24 and 1/20 (although it did not stop the manufacturers from rebranding their 1/28 kit as 1/24, especially after mergers and buyout of molds from bankrupted kit makers).
By the 80's the market has pretty much settled on 1/24, with LS and Bandai as the last holdouts for 1/20. After Bandai stopped making model cars and LS went out of business in the early 90's, 1/20 finally died off as the general car model scale in Japan.
Tamiya started out with 1/12 for their detailed car kits, and scaled down to 1/20 in 1977 with the Tyrell P34, and to 1/24 in 1978 with the Porsche 935. By that time the Japanese market had already settled on 1/24 and 1/20, and I don't think there would be any Monogram influence that late in the game.