Transaction replacement is so much a part of what Bitcoin was intended to be that Satoshi included a 32-bit field in every single transaction solely to provide ordering for replacements. (It was disabled only because the original design let people spam the network with unlimited replacements for free, which was later fixed by requiring replacements pay higher fees.) If you want to accept 0-conf transactions, you just have to check they're not marked replaceable - and the way you check is the same as in Satoshi's day.
What does break 0-conf transactions is the transaction malleability problem that Segwit was designed to fix. It's not safe to accept 0-conf transactions from people you trust entirely, or even from yourself, because third parties can invalidate them by malleating the transactions they depend on. This majorly broke outgoing payments from some Bitcoin Cash exchanges shortly after the fork and required manual intervention to fix. Satoshi's original code was even less able to cope with malleability; it got completely confused and wallet transactions tended to get permanently stuck.
Also, the new difficulty adjustment algorithm in the 1st of August fork was terribly broken. It had a design flaw that allowed miners to repeatedly drive difficulty down much lower than it should be in a relatively short period of time, then mine large numbers of blocks at the lower difficulty, creating new Bitcoin Cash at a much more rapid rate than intended. Not only that, but during the drive-down phase blocks took hours to mine, making the network almost unusable a large chunk of the time. That's why exchanges resorted to using change from unconfirmed withdrawals as inputs to subsequent withdrawals, causing chaos when someone exploited malleability. It's also why supporters were so keen on promoting 0-conf transactions as safe.
On, and the tweet you're pointing to as proof that there was no discussion of limiting the block size until the evil Blockstream got involved was written by one of the key employees and co-founders of Blockstream after he founded it. Meanwhile, the most ardently small-block of the core devs is Luke Jr who's very conspicuously refused to work for them in order to preserve his independence.