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.
With only a throughput of 7tps, it definitely sound like a limitation. Nevermind 2nd layer solutions, we're talking about onchain transactions.
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.