Same brand, different battery
HART also sells HART 20V. We have no source saying its packs fit HART 40V tools, so don't assume a shared brand means a shared battery.
The HART 40V lineup
HART 40V is HART's cordless battery platform. It is no longer sold new. PowerToolGrid lists 15 products in it: 6 batteries, 3 string trimmers, 4 leaf blowers and 2 chainsaws. Sold only at Walmart.