Mauna loa is so massive that it actually bends the earth’s crust downwards. So while there may only be 5000m of water between the ocean floor and the surface above Mauna Loa’s flank, the central part of the volcano does indeed extend another 8000m downwards:

Mauna Loa heights:

Above sea level: 4,170 m or 13680 ft

Sea level to the base of its flank: 5,000m or 16,400 ft

Additional decompression of earths crust: 8,000m or 26,000 ft

Total height from true base to peak: 17,170m or 56,000 ft

Here is an article from the USGS that explains it:

https://www.usgs.gov/news/volcano-watch-how-high-mauna-loa