Survey report- 1920s terrace with no roof underfelt

The survey indicates to have this inspected by a roofer as underfelt should have been added when the roof was renewed (unclear to what extent yet- solicited to investigate). FIL works in roofing and says without underfelt it's a disaster waiting to happen, and adding it would be costly as involves taking the whole roof off. Doesn't understand why it wasn't added when renewed. MIL (unasked for) input that we would be stupid to buy it as is. Surveyor was positive about the condition of the house and said it was well maintained given its age. FIL thinks we need to renegotiate price in order to get roof fixed.

Googling has me confused - lots of results saying underfelt isn't necessary and there's if no water damage (none seen but surveyor indicated further investigation by roof specialist to be sure) then the tiles are doing their job currently so it's not the sellers problem.

Is this just a risk of buying an older terrace house, that we have to either be willing to accept could be a future issue or is it something the sellers should be fixing before selling?

We are very confused!