In fact, this is about "tongue" not thumb sucking.
I found it interesting - the analogy that perhaps in modern, well nourished
women thumb sucking may be a way for a child to comfort themselves without
getting more milk than they need, because the only child of three that I had who
sucked - his tongue, with a very loud noise - was the one who nursed and nursed
until he threw up, then returned to nursing.  I finally (reflux was not
diagnosed then - 1974) went to limiting him to one breast for three hours - he
was allowed to nurse whenever he wanted, but at one breast.  After three hours,
I would change to the other breast.  He was also my "fattest" baby, so there was
no weight gain problem which would indicate I should be feeding him more.  He
was the child I thought would nurse until he went away to college - but refused
to nurse when my milk changed to colostrum during my third pregnancy.  Tongue
sucking continued for many years (only when he was falling asleep) later.
Even though every child's perception of stress in the family is different, I
don't think he was under any more stress than his older or younger sibling - we
were not moving or having any changes at that time (which we later did!).  He
just needed to suck his tongue.

Jeanette Panchula, BSW,LLLL, IBCLC, RN (that was the order I earned them!)
Puerto Rico