IC doesn't yet support source-tracked sandboxes. It's something I'll look into this week most likely.
Scott Wellsrepo owner
FYI, I've now implemented this and am in the process of testing it. It won't go into the next build because I'm going to be out of pocket most of next week. However, if you're interested I can post a build with it here for you (and anyone else who is interested) to try out and provide feedback. Otherwise I'll likely release it week-after-next.
Issue 1676 - Added support for push/pull against source-tracked sandboxes. To enable this behavior for a sandbox, check Use push/pull instead of deploy/retrieve/delete in the sandbox org's connection settings. When enabled, the sandbox will behave exactly like a scratch org with push and pull as the default behavior for metadata operations (including deploy-on-save) and raw deploy/retrieve/delete operations available as explicitly-invoked actions. The environment variable SFDX_ENABLE_MULTIUDX is automatically set to true for all push/pull operations against source-tracked sandboxes.