I don't think its fully reliable. That is good only to get a range. If you are extremely good with fitting data, or accurate with exact weight loss with every temperature range of other components (Hemicellulose/Cellulose), than probably this can help. Lignin has wide range of temperature for decomposition.

If your sample has only lignin, then yes, its reliable.

But still, Its good to find out lignin content with standard method to validate.

Thanks Ankita Gupta !