I saw the original production in 1970 with Merman, and the most recent Channing revival.
I don't remember that bit of business in either of them (but who knows?).
That bit wouldn't make much sense without the song.
The "bit" is not in the script of the original stage production and , until the current production, I have never seen it performed.
FWIW, I think it is charming. It also adds a touch of sincere sentiment to Horace's rather profound and "sudden" change of heart regarding marriage to Dolly.
