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If successful, it would split miner resources and cause the next block to be mined at half speed.

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and split the network 50/50...

I am aware that is what he meant.

Then in that case, I think it would cause the next block to be mined at half speed.

No.

Imagine rolling dice. I give 1 person 2 dice and ask them to roll the dice once per second until they roll at least one '6'. Will it take twice as long (half the speed), if I instead "split the network" and give two people each 1 die, asking then to roll their die once per second until either one of them roll at least one '6'?

Same number of total dice, same target, same average time to success.

With bitcoin: Same total hashes per second, same target hash value, same average time between blocks.