Now that I've read Roche's PR, it all makes sense. The threshold for Roche to continue is a simple majority of the 44% that Roche’s doesn’t already own (i.e. 78% of DNA’s total equity).
The significance of the 90% threshold is in allowing Roche to bypass a DNA shareholder vote and proceed via a short form merger. But this is a matter of timing only—even if Roche ends up with less than 90% of DNA’s total equity and has to hold a shareholder vote, Roche can vote its own shares and make the merger a fait accompli.
“The efficient-market hypothesis may be the foremost piece of B.S. ever promulgated in any area of human knowledge!”