A bunch of BS in the airline response. Airbus and Boeing have nothing to do with it if some 20 year old plane needs seat maintenance. The seats are airline customer items with separate manufacturers around the globe with hundreds of different models of seats available which airline chooses which one they want to use and all spare parts and maintenance is either the seat manufacturer or airlines responsibility especially on 20 year old planes.
The problem might be that nobody has spare seats in stock with the exact thai airways specific colors and sprinkling in some seats with different airline colors looks very bad.