Just finished a matchup where I made that last two matches, the second match of 3 chained through all 3 characters HP without breaking. I understand the scenario is probably tremendously rare but should this really be possible?
Yes.
The damage will get reduced to next to nothing at the end of a chain, though in your case the opponent probably got a match 5 so he can start again and technically this cycle can indeed go on indefinitely.
Especially fun when the enemy venom chains 5s together with his whole team stun