I have two theories, one that I believe is more likely than the other.
We'll start with the less likely one:
Rand al'thor was able to light the pipe by manipulating chance as a Ta'veren. In a previous book, he made a threat to Cadsuane Sedai, saying that he could stop her heart through chance by willing it to be so. He could have lit the pipe by willing it to be so in a similar manner. However, I believe this is likely: Rand should no longer be a Ta'veren after he defeated the Dark One.
I believe my second is more likely, though it has very strong implications:
When he was battling the Dark One, he was able to manipulate threads of the Pattern itself to show what the world would look like after killing Shai'tin. Additionally, he was able to channel Saidin and Saidar in their purest forms. Since he can no longer channel weaves of fire directly, he lit the pipe by manipulating the Pattern itself through his will. This leaves him as immensely powerful, far morso than any channelers that remain alive, though I believe he would choose not to exercise that power in the future.
