[spoiler=shishio]That is not true. I won't say anymore, although if you want to know what actually happens, see the next spoiler. It is not nearly as gross as you describe, though.[/spoiler]
[spoiler=Shishio, serious spoiler content in this one!]What happens is that, since Shishio's burns destroyed his sweat glands, he can no longer regulate his body temperature. Therefore, he can only fight or perform vigorous physical activity for 15 minutes before his body temperature becomes dangerously high. In addition to being beaten by Kenshin, Shishio keeps fighting long past this time, so his temperature becomes so high that the fat in his body ignites, comsuming him in flames. It is not gross: he just thrashes around covered in flames. There is no explosion, and you do not see any blood or internal organs.[/spoiler]
I hope that helps.