Hate to bring back an old topic, but I found a permanent, MacOS-based solution to this after speaking to the wonderful senior support at Apple (h/t Nick at Apple, 1109457).
To force folders in Alfred searches to open in Finder tabs:
Go to Apple > System Preferences > Doc
Set "Prefer tabs when opening documents" to Always.
I'm doing this on on macOS 10.14.3 (18D42). Works like a charm.
You are welcome.