Force ios style in select popovers?

I’m trying to force the ios style (checkmarks instead of radios) in an ion-select popover.
We’re on Ionic 3.9.2;
In the closed Issue #10531, discussion seems to indicate I should be able to specify it as follows:

<ion-select interface=“popover” [selectOptions]="{ mode: ‘ios’ }" >

But, it does not seem to work - I still get the “md” popover with radio buttons.
Any clues? Is this supported in v3.9.2?