SMPPS can be hopped, but not in the same way that proportional can. Proportional is hoppable because the expected reward per share is different at different times, so hoppers mine when the reward is high (early in a round) and leave when it is low.
In SMPPS, the expected reward is in theory constant, but the maturity time, the time it takes to actually receive the reward, is not (maturity time is bad because of the time value of money, uncertainty wrt the future of the pool, etc). When the buffer is positive, SMPPS is very attractive, as it is like PPS but with 0 fee. When proportional pools become extinct, hoppers will choose to mine in SMPPS when its buffer is positive because it is better than alternatives. When the buffer is negative the maturity time is high, so the hoppers will leave, and the honest miners will suffer from more than their fair share of the bad times.
So, the hoppability relies on the possibility of a negative buffer. But this possibility is very real - the underlying stochastic process is similar to the one in PPS, but the fee which helps PPS stay afloat is absent. This means that with probability 100% it will become negative at some point, no matter what the initial buffer was.
See also Analysis of Bitcoin pooled mining reward systems for more information about reward systems.