Unless you can point out where the statistics say that, your belief is just speculation. I've skimmed over the data and didn't see anything about attributing depression to work problems. (If you have other sources regarding this, please link them.)

Table 2-3 shows the breakdown of "health problems," and depression is at about 40%. Even if all of those were due to overwork, this is only 40% of the "health problems" category, which is not enough to support a claim that "most suicides come from excessive work." For sure, there may be many cases that do, but there are no clear numbers in that regard. Personally, I feel such claims only reinforce the typical stereotypes about Japan rather than try to understand the actual situation.