I’m certainly not a nuclear engineer, but I have read descriptions that said the Russian reactor was literally running in a common cement building/ warehouse, with no containment or safety systems whatsoever.
You are correct in that there was no containment vessel for the Russian reactor design. As theoretical as Soviet nuclear physicists were, they hung their hat on outdated reactor technology. Remember they stole their tech from the US which built the last type of this reactor in 1962-63 at Hanford. This was the “N” Reactor with a primary mission of producing nuclear bomb material. It was decided shortly after to build the “steam generator” plant near it to capture the steam to produce electricity.
The N Reactor was operating just fine until Chernobyl at which time the Democrat Senators from Washington and Oregon had their own meltdown and caused enough uproar that it was shutdown.
I was a pipefitter at that time and had worked on the N Reactor and on several modern PWR and BWR reactors and the technology and level of building safety was night and day between N and those new reactors.
What made the Soviet reactors a danger was that each station had their own chief physicist who was allowed to conduct experiments to test the capabilities of their systems, which is what caused the accident at Chernobyl. Basically trying to see if they could generate power during shutdown operations until the diesel generators would kick on and operate the water pumps. Apparently there was a minute or so delay from between shutdown and diesel startup.
I have read a few books on Chernobyl. It was an old reactor design with no containment vessel. They has already had incident with a RBMK reactor outside of Leningrad. Add poor materials, poor construction, arrogant scientist, and bureaucracy that cares little about anything except results and it is a disaster waiting to happen.