My BASIC memories are rusty, and more oriented toward Applesoft than Microsoft. But I remember STEP being used to give an optional increment to the FOR statement, rather than setting a line numbering increment - for example, you might

    FOR i$ FROM 1 TO 10 STEP 2
    PRINT i$
    NEXT

and get back 2, 4, 6, 8, 10, each on its own line.

I don't remember a command to set the increment for line numbers, but that may be because I never used a BASIC which didn't either require a number on every line or not care about line numbers except as targets for GOTO and similar.