In AppleScript can you create a window tab without using keystroke?
I can say until 10.9 this isn't possible because the property of tab isn't implemented into applescript. After 10.9... I don't know but you can check it by yourself like this:
Open 1 Finder Window with two or more tabs, then:

tell application "Finder"
properties of Finder window 1
end tell


...now if you can find something in the result that speaks about tab, then you can use this property to work on tabs. Alternatively just open the Applescript Finder Library and search for tab. It would show you if there is a property.

Also...

tell application "System Events"
keystroke "t" using command down
end tell


wouldn't be necessary if property tabs would be implemented into Applescript.