It is important to understand that Atticus puts together a bunch of details and believes Jem must have killed Bob Ewell. This is Atticus' righteous character coming through and even though he is almost accusing his own son, he believes it needs to go to trial. Heck wouldn't have had to explain much if he didn't have the need to correct Atticus.
In correcting Atticus, Heck (the reader knows) is actually protecting Boo Radley. Radley killed Bob Ewell in defense of the children and rescued the kids. There was a struggle between Bob and Jem, Boo intervened. Bob Ewell must have had a knife on him with the intention to kill the children and Boo must have wrestled it away and shoved it up under Bob's ribs. Then, he carried Jem to the house as Bob was left, dead body, laying in the street.
None of this is told directly to us reader, Scout reports the sounds she hears because it was dark.
Heck put togther what happened and decided it best to report that Bob tripped and fell to protect Boo from the town bringing him gifts and praising him for getting rid of the bad guy. On the flip-side, even though Boo did something morally right in protecting the children, it is indeed legally wrong to kill. Who knows what could have happened if that went to trial.
So, does Heck Tate lie? Yes, in that he didn't tell the truth that he figured out. But also NO, because he didn't see the crime. He can't for sure know exactly what happened.
Sheriff Heck tells a story about how Bob Ewell had been drunk and fell on his knife causing his own death by accident. He is aware that if he puts Boo Radley's name in the mix, the people will put all their suspicions together and convince themselves that Boo had harmed him on purpose. Boo would not get a fair trial because he would be as much of a victim ad Tom Robinson had been. Heck Tate wants to save Boo since he could not save Tom from the unfair trial.