Why not to add a 'slow' CPU mining option to the full-node software?
Because it is more likely that you will win a millions or billions of dollars in a lottery than you solo mine a single block on your CPU. Also, this option is still available, you can call generatetoaddress and mine a block in this way. Also you can connect any CPU mining software, choose SHA-256d as an algorithm and start mining on your CPU. Technically, you can connect your CPU miner to your full node. Practically, you will get nothing, or maybe a single block in your lifetime, if you will be extremely lucky.
In practice, if you want to reintroduce CPU mining option and if you want to get any coins, you can try to do some kind of mining in the Lightning Network. Then, I can imagine that a thousands or millions of CPU miners will mine on some shared N-of-N multisig, expressed as a single taproot address, and maybe will mine some block together. But for most of the time they will get nothing and use a fraction of their computing power to replace LN fees with their blocks. So, it is technically possible to, for example mine a block way easier and instead of receiving 6.25 BTC plus fees, receive one satoshi in LN, because your block will meet around 625,000,000 easier difficulty and another party will accept shares instead of coins.
First of all, how would consensus work when are 2 different PoW (SHA-256 and other algorithm for CPU) on the network?
Currently, changing the mining algorithm from SHA-256d to something different is not needed. All that is needed to decentralize mining, is receiving smaller amounts of coins without having to trust any centralized mining pools. I can imagine people moving single satoshis, millisatoshis, or maybe smaller amounts in decentralized way. The algorithm can be the same, as long as it is not broken.
No no, I think he means simply adding back the CPU mining capability to Bitcoin Core.
Nobody needs that. You can connect any miner to your full node, in this way you can replace the old miner with the new miner (and even run them on two machines in parallel to compare them) without changing Bitcoin Core. That situation is way better than rebuilding the Core client, just because you want to make faster miner.