Not sure I can agree. The Klingons can be full of it, but I doubt they’d send a shuttle or even just a scoutship to challenge a Federation heavy cruiser, given the Klingons themselves have referred to a Federation heavy cruiser as a battle cruiser (TSFS). In “Errand Of Mercy” a scout type vessel did attack the Enterprise in a surprise attack, but that seemed to be due to surprise and maybe more than a little Klingon hubris.
The situation conveyed in “Friday’s Child” is that this is a vessel capable of making a serious challenge. If the D7 miniature had been available then it’s likely thats what would have been used. But apparently it wasn’t so they cobbled something together that looked undefined at extreme range onscreen. We can speculate on whether the D7 miniature could have been made available sooner if Jefferies had already designed it or we can speculate the Klingons have another piece of heavy hardware in their inventory. The first possibility is the most cost effective in the long run to have one miniature that can be reused. The second possibility is more interesting because it adds more texture to the TOS universe.
For the second possibility to work, given we know the D7 will eventually appear, is to have something that could have been quickly cobbled together and not need to be heavily detailed as it will be shown only “at extreme range.”