[thescriptkiddie]

I'm not the person you replied to, but I can answer this.

Segwit is technically a soft fork, which means that it has some degree of backwards compatibility. Segwit will be redefining a previously-unused opcode in the bitcoin stack machine, which all old clients should treat as a NOP. So old clients will continue to operate normally with old transactions, but they will see segwit transactions as junk and be unable to validate them.

In the past this hasn't been much of a problem because soft forks have always implemented optional features. However this time it will be a problem, because the plan is to move all transactions over to Segwit. Any client that hasn't been upgraded once Segwit is activated will likely see no transactions at all, the same as if there had been a hard fork. So that's why they added a 95% activation threshold.