A small correction. HOT stands for Heap Only Tuple.
Right. The text is based on stale information. HOT stands for Heap Only Tuple and the restriction about not changing tuple length during UPDATEs has been removed. HOT optimizes a special but important case where UPDATE does not change any index key columns or columns referenced in index expressions and predicates. HOT helps early reclaim of dead space in a table, thus reducing the need for frequent VACUUM and reduces relation and index bloat.
I would suggest the following README to know more details.