Yea, this is a terrible way of calculating this. I would like to see it done based on capacity (which sadly isn't public for part-ownership).
It kinda is. Interestingly a lot of these seem to be divided up by fiber. E.g. for CAP-1 it's a 6-pair system, FB/Meta own 5, Amazon owns 1.
This is from https://www.submarinenetworks.com/en/systems/trans-pacific/c...
I'm not sure what their source is though.
Calculating fiber pairs -> bandwidth is much much harder though and is an implementation detail which will absolutely change over time. I have no doubt that part of why FB/Google/Amazon want to own fiber this way (owning dedicated pairs) is so they can experiment with different WDM technologies.
Their capacity for failure in the overall system is far higher than the traditional companies who are selling circuits and must design things for many nines of reliability.