Nobody has run the trials on severe depression
The reviews that establish it works in mild and moderate depression say plainly that severe depression is under-researched, and that whether it helps people at high risk of suicide is unknown.
What the study found
Severe depression is the question nobody has run the trials on
Absence of study is not a negative result. The reviews that establish St John's Wort works in mild and moderate depression state plainly that severe depression is under-researched, and that whether it helps people at high risk of suicide is simply unknown.
The caveat · US trial cited was moderately severe
The large US trial most often cited as proof that it fails in severe depression enrolled people its own authors described as moderately severe, so it did not test the question either. No trial has been designed with severity as the thing being varied.