Earlier quoted context omitted.
You're forgetting that every miner chooses how they mine. Someone with 51% hash power can decide to always mine on top of their previous block, ignoring what everyone else does. And because the 51% chain is longer, the 49% chain get discarded by any miner following default bitcoin rules. So the effective block rate gets slowed down, and the person with 51% of the power has control of 100% of the canonical blocks.
This would make the 51% miners' chain grow at half the speed as the 49% miners' chain, because the 49% are building on both their blocks and the other miners' blocks, whereas the 51% are only building on their blocks. This, of course, is assuming that the 51% doesn't create blocks considered invalid on the other chain. This would cause a hard fork and it's exactly what is happening now with Bitcoin Cash, except with…
They are building on some of their blocks and some of the other miner's blocks. This doesn't make them faster.
Whenever a 49%er mines a block, there is a temporary fork. The 49% and the 51% start mining on two completely separate branches, both of which progress at half speed. Eventually the 51% branch is longer, and the 49% discard their previous work and switch to it. So the 49% were using their blocks, and now they are using the 51% blocks, but never both at the same time.
The canonical blockchain does not progress at full speed. Half the blocks that get mined eventually become orphans.
It would be impossible for the 49% to build on everyone's blocks, because the 51%-created blocks are always mutually incompatible preexisting 49%-created blocks.
> This, of course, is assuming that the 51% doesn't create blocks considered invalid on the other chain.
Yes, of course.