As you point out Lightmass is very intensive process. I’ve not personally gone through the scene aside from looking at the video and stills so I’m not 100% positive on some of this, but some things to check.
This is a Architectural Visualization and typically they will use higher lightmap resolutions along with some settings changed in World Settings > Lightmass to get better results. Often this will be lower the Static Lighting Level Scale, increasing the Indirect Lighting Quality, and other settings that will increase light build times along with make them use more precise calculations which in-turn uses more resources and memory.
The solution here if you’re running out of memory would be to increase the RAM in the computer or lower the settings back to default. Lowering the settings will not result in the same baked shadow quality though.
One final alternative is to increase your virtual memory cache. This will allow lightmass to continue when it runs out of RAM and write directly to the hard drive, but this process is very slow and will take longer to build lighting, pending it doesn’t run out of memory there.
I hope this helps.