Well, this 80% efficiency is what they are targeting not what the system will do. The test system Energy Vault build in a MUCH smaller form factor had a round-trip efficency of 75%

The EVx ™ system is projected to achieve an impressive round-trip efficiency exceeding 80%. Source

Only time will tell if they can reach 80% with a bigger system or at all. If they actually manage this it would be a decent alternative to Hydrodams in areas where these are just not possible since it would be a similar round-trip efficency.

Pumped storage systems have a round-trip efficiency of about 80%, which is competitive with battery storage. Source

In my opinion these systems are inferior to fly wheel energy storage (can reach up to 90% round trip efficency Source) but might still be an option depending on price.