tr:dr; he says "x86 took over the server market" because it was the same architecture developers in companies had on their machines thus it made it very easy to develop applications on their machines to then ship to the servers.
Now this, among others he made, are very good points on how and why it is hard for ARM to get mainstream on the datacenter, however I also feel like he kind lost touch with reality on this one...
He's comparing two very different situations, more specifically eras. Developers aren't so tied anymore like they used to be to the underlaying hardware. The software development market evolved from C to very high language languages such as Javascript/Typescript and the majority of stuff developed is done or will be done in those languages thus the CPU architecture becomes irrelevant.
Obviously very big companies such as Google, Microsoft and Amazon are more than happy to pay the little "tax" to ensure Javascript runs fine on ARM than to pay the big bucks they pay for x86..
He is sort of right, back in 2019. Even then, IBM PowerPC mainframe are still thriving.
Now, new language with cross compilation with some maturity are here. Major cloud providers now have ARM base machines ready, even designing to their own need.
ARM is in the datacenter market and become a trend.
The only thing I worried about, is the architecture of ARM are too fractured. AWS Graviton might behave differently than Ampere Altra, despite both have the ARM ISA.
With x86, there are AMD and Intel. With ARM, how many designers are here? With more designers, the smaller the potential common ground is, and more code paths to optimize, thus cost more to build.