Again, see
Hull Note presented earlier, here's the interesting parts:
Color:
Grey: Probably not binding, notice that they used "will endeavor to V" instead of "will V"
Green: Favorable to the US
Yellow: Favorable to Japan
White: Appears to be rather neutral
Though 2. is definitely pro-US, and I assume Japanese economy was way more fragile in 8.
We're talking about a Japan that didn't further invade China after 1933 nor snatched Allied countries' colonies. Unlike IRL, Japan likely didn't commit as nearly many war crimes nor bombed the US, and for the US, Japan would be "an enemy of an enemy of my enemy", far from "enemy". Japan also had no juicy colonies up for grab in convenient places, unlike the earlier wars with Mexico and Spain.
In reality, any military vessel is fast enough to board a freighter - there's no need to shoot. And Japan could form a line outside of every ports in the Russian Far East, unlike Germany, so there's little chance someone could make it through even if no shot is fired. Though it's not going to be in the Cuban way, there must be hostilities between Japan and USSR for Japan to seal the waters off, telling everyone that this is a warzone that's offlimits. If the freighters were turned back or towed back at the blockade line, it's no more insulting than arresting neutral citizens who tried to walk through an active trench line and sending them back.
Because Japan and the dominant faction within the military opted to stay peaceful with the USSR with their Southern Plan and gave a giant middle finger to anyone trying otherwise, hence the basically-almighty commander of the Kwantung Army was sent to reserves for his aggression against them. Kodoha had fallen in 1936 so there's no one he could seek help from.