This is such an underrated comment. Not being afraid of your customers and establishing communication goes a very long way especially when you're starting out.
That's a good thing to do anyway, but keep in mind that your customers are not a random sample of your desired customers.
I regularly keep in touch with some clients, but they are unlikely to tell you how to get people to buy a subscription :)
Many will tell you what they think it's worth, and if the app went away (even with lifetime), what they would be willing to pay elsewhere.
Asking lifetime customers may not provide a full picture though.
I think they meant to say you should directly ask those customers why they didn't buy a subscription.