Correct, I don't think that's possible, or even desired - I don't think
Strava would want 3rd party apps to be able to sign their users out of
Strava. But I think there is a suggestion to Strava in here somewhere:
the authentication page that lets you sign in to Strava doesn't provide
any way to sign out and sign in again as a different user - it's quite
common for these pages to contain a link to let users do that, making
the process of switching users when signing in to 3rd apps much
quicker/more streamlined (e.g. see Twitter's oauth page, and similar).