But something change in the difficult of testnet in the last hours, somebody is mining like 5 blocks per second, so if someone wants to learn how to mining they can try Testnet on this momment.
I don't see what's strange about that, as there is no return from testnet mining. There is rarely competition for block mining, and thus one miner with good hardware can mine several blocks after each other.
testnet will not teach you or enable you to understand Bitcoin mining because of the difference between many things between these two networks, but it is good in development and the evidence for this is its role in the CVE-2018–17144 double spending vulnerability.
https://bitcoinops.org/en/topics/cve-2018-17144/#:~:text=CVE-2018-17144%20was%20a,included%20in%20Bitcoin%20Core%200.14the bug was exploited on Bitcoin testnet, causing any nodes still running Bitcoin Core 0.14.x to crash and nodes running 0.15.0 to 0.16.2 to accept a transaction that spent the same funds more than once. When testnet miners managed to produce a valid chain with more proof-of-work than the invalid chain, those non-upgraded nodes attempted to switch to the new chain but were unable to fully un-spend a double-spent input.