It is like that because if more miners join the network, the mining hashrate increases, and this will lead to transactions getting confirmed more below 10 minutes. But if miners stop their miners from working and are not mining, the hashrate will reduce and transactions will be getting confirmed more above 10 minutes.
This statement is misleading. There is no direct correlation between the transactions in the mempool and the average block time. It would be entirely possible for the average block time to be 12 minutes with an empty mempool resulting in every transaction getting in to the next block and getting fairly rapidly confirmed, and equally it would be entirely possible for the average block time to be 8 minutes with a packed mempool and the majority of transactions waiting several hours to be confirmed.
Which means that transaction confirmation can take less than 10 minutes or more than 10 minutes, but will be almost 10 minutes on average if the average of the total transactions are calculated.
This is not accurate either. If the mempool is full, then the average transaction time absolutely won't be 10 minutes. The average block time might be 10 minutes, but if ~2,000 transactions from a pool of 100,000 are included in each block, then the average confirmation time will be much longer than 10 minutes.