I think its between 50-75% of those who interview get an offer, depending on applicant pool size, degree path (like MTS vs MDiv vs MRPL), and funding. Previous years had about a 25% general acceptance rate into Harvard Div, with odds steadily decreasing due to an increase in popularity (I think acceptance used to be closer to 50% in 2012). And in tracking attendance at various open house events and informational panels, it seems like this year will have the most amount of applicants they've ever had. So the number may lean closer to the 50% side than the 75% side when it comes to odds after interviews.
One of my friends who works in admissions at another school said they also started doing mandatory interviews due to the increased use of ai in essays to screen for three things: 1) to see if the candidates are an accurate reflection of the content of the material they submitted, 2) to see if the candidates match the caliber of the material they submitted, and 3) to see if the candidates can think on their feet. So as long as you're able to do all three well, you can hope you've done alright!