Unless Song Tianmo and Ling Fei divorce, he... can't get engaged to another woman.
But the Song family didn't know about her marriage with Song Tianmo. Old Master Song seemed extremely serious when he said this, not like he was lying.
If the old man wasn't deliberately misleading her, then that means... the Song family really has chosen someone else for a marital alliance!
They wanted Song Tianmo to marry the person they liked.
Ling Fei's relationship with Song Tianmo was legitimate.
Although this legitimacy was something she had fought hard for herself, no matter what... since she had fallen in love with Song Tianmo and wanted to stay by his side forever, she had to make the Song family accept her too!
"Grandfather, we live in an era of free love now. Although today is the first time we're meeting each other, just by looking at you, I can tell that you are an enlightened and reasonable person, so you should know it's not wrong to like someone!"