If segwit activates, any attempt to steal anyone's Bitcoin by stealing segwit outputs will be considered invalid and any blocks that contain said transactions will be invalid and thus will be a hard fork. With a hard fork, anything goes. Anyone can make a hard fork and confiscate anyone's coins on that fork, it is, after all, a change in consensus rules where you can make the consensus rules whatever you want. "The Real Bitcoin" is no different. In fact several forks have been made from Bitcoin which no one really cares about, and "The Real Bitcoin" will just be another one of those. If there is no economic activity and no users actually use a coin, it is worthless and no one will care about it. Currently ALL miners are signaling for segwit per the BIP 91 rules. No miners are using "The Real Bitcoin" and no major businesses, exchanges, or users are using it. In fact, the vast majority of users (including businesses and exchanges) and miners are supporting segwit.
So sure, you could hard fork and steal coins spent in Segwit transactions, but no one would care because it would be a hard fork and it would just become another altcoin that no one ever thinks about.
Also, you could just steal all P2SH coins now. P2SH was released after "The Real Bitcoin" made their fork. And it would get you more coins much sooner. But no one would care if you did.
Miners are also incentivized to not hard fork and steal coins. Besides the fact that stealing coins means that they will be damaging the value of the chain they forked from, miners are also then taking the risk that the fork that they switched to will be completely worthless. It is far more profitable to continue to mine the chain which the majority of the community backs and will be using than it is to mine some fork which will likely be worthless with no users on that fork. There will be more transactions on that chain and more value with it as people are actually using it. Furthermore, any miner who did choose steal anyone's coins would come under significant criticism from everyone in the community and that would be terrible for their reputation.