This is not an issue because that was the nature of bitcoin in the first place, the more blocks are getting mined, the more the next blocks are going to be harder to be mined.
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I'm not sure what you mean by the statement by the way. Difficulty is a function of the average time difference between each block.
I'm also confused with how the next blocks will be harder to mine. He probably mixed it up with the more no. of confirmations, the harder it is to reverse. If that's not the case, maybe he meant mining a block before and after the difficulty adjustment?