No, it was not. That map only shows the area of responsibility of the various armies, not the region covered by the defensive redoubt. This map, on the other hand, does show that:
The redoubt is the big circle in the Third Area Army's sector, well to the northwest of Sinjuin. There is a forward fall back position at Sinjuin (it's the little semi-circle you can see there), but the map indicates the defensive positions face due west and leaves the direct northern approach to the city uncovered.
Interesting, because the Soviet record of their advance shows they captured the region of Ranam-Guyok, which is to the southeast of Chongjin in that time period. Were the "previously prepared defense positions" to the south of this region? Because if so, then that would contradict the quotes statement they were still in engagement with the enemy. The fact that the quote states General Nishiwaki subsequently had to order a retreat another 55 miles to the southwest on the morning of the 18th, before he even heard about the surrender, likewise contradicts the claim Soviet forces were being held.Also, according to the JM-155 monograph:
"Nanam Divisional District Headquarters had lost contact with higher headquarters due to the severance of communications, and was unaware of the Imperial Rescript of 15 August. Its force continued to engage the enemy from the 16th to the 18th and by holding previously prepared defense positions, it checked the enemy's southward advance. On the morning of 18 August, General Nishiwaki decided to move his troops to the vicinity of Kilchu, about sixty miles south of Chongjin. At about 1800 hours, while en route, a staff officer of the Korea Administrative Defense Army joined the retreating columns, bringing word of the cease-fire order."Effectively, the Soviet advance into Korea was blocked on all routes.
Or were perhaps these prepared defensive positions more to the north/northwest, due west of Chongjin? Because if so, then the various elements of the story can be reconciled and the retreat makes sense, because Nishiwaki needed to do it in order to avoid getting outflanked and cut off. But it still contradicts the claim that the Soviets were ultimately being held.