In the summer of 1940 Japan was buying all the oil it needed from the US
This makes it sound like the Japanese were completely oblivious to the strategic significance of the DEI and the resources in them up until the minute the UW froze their assets and embargoed oil. However the DEI was a planning factor and a (long-term) objective of many Japanese navalists before 1940.
was up to its neck in China
It was up to its neck in terms of occupied territory, but wasn't using all its arms and legs for China oriented offensives. For 1940, the Japanese had scaled back their offensives in China and didn't do any campaigns on the scale of 37, 38 or 39 that year. Meanwhile, the Nationalists and Communists both did their own general offensives but those didn't do that well.
was not yet confident the USSR would stick with the defacto truce on the Manchurian borders.
True- the biggest limiting factor. Yet at the same time, the USSR had already initiated negotiations for a neutrality or nonaggression pact. Also, with the Fall of France it might have occurred to Tokyo that the USSR would have to hedge more against a possible failure of the Nazi-Soviet Pact and likely would become more cautious.
There are some minor technical details as well, such as the Japanese had no naval bases in effective range of the DEI.
Their naval and air bases in Micronesia ranged the eastern part of the DEI archipelago. Hainan ranged Borneo. Throw in some carriers and you can project more power, and use captured bases to extend your naval and air range.