I decided to approach this issue from the other side. I.e., from the Japanese side. Surely, if something like that really happened, there has to be some sort of Japanese record of it.
I didn’t find any mention of Rubitsky, but that’s not entirely surprising. The main problem I had is that the information provided in the Rubitsky link isn’t terribly accurate. For instance, it doesn’t mention where the triangle was.
Nevertheless, I did find references to officers by the name of Yamamoto and Yasuda active in New-Guinea. Note, however, that those are very common surnames. Note also that English names for military ranks don’t translate all that neatly into Japanese.
With those caveats, Commander Yasuda Yoshiatsu born 1897 in Hiroshima died in battle at age 45, on January 2nd 1943, in New-Guinea.
I couldn’t find exact dates for Colonel Yamamoto Shigemi (the most likely candidate), but this here site lists his year of death as 1943, not 1942.
Col. Yamamoto is mentioned here also. The comment is particularly interesting and also a bit frustrating. It mentions a death note left by Yamamoto singing the praise of the Takasago-Giyu regiment. The site also mentions that the note was widely known of amongst soldiers fighting further south. What is particularly interesting, though irrelevant to the subject at hand, is that the Takasago-Giyu regiment was made up of native Taiwanese soldiers.
Moving along, this page traces the development of the “Kokoda battle”, which is probably the one we’re interested in. I’ll give a translation of the relevant passage:
If we are to believe this site, Col. Yamamoto died with Commander Yasuda on Jan. 2 1943.
From my gleaning of Japanese sources, there seem to be great discrepancies with Rubitsky’s tale, at least as far as dates are concerned.
Note, however, that Japanese commanders did indeed, at this point, consider the fight for New-Guinea lost. Troops were starving and badly supported. So the alleged suicide note was accurate on that point.
I’ll try to find more if I have time.