TA wrote:"Hitler didn't use chemical weapons" was definitely a scripted talking point, and that I think was an intentional dog whistle. "he went into towns, dropped them down to innocent- into the middle of towns" was Spicer talking scriptless and with no prep time. That was him talking off the cuff, and that's when internal prejudices really come out, when you don't have the time to internally censor what you're saying.
Spicer himself might or might not be a Holocaust denier, but I think it's clear he thinks that whether it happened or not, it would have been the right thing to do.
I read it more as Spicer doing what he always does: somebody points out that what he just said is obviously wrong, and he bullshits his way into explaining why it's actually right. That Hitler's methods were different from Assad's is of course true, so Spicer fumbled for that and pretended the differences were relevant to the matter at hand. Very, very badly. ("Holocaust Center." My God.)
All that said? Dude works for Trump. Excusing xenophobia and fascism is basically his entire job description. "What percentage is Spicer antisemitic and what percentage is he just stupid and ignorant?" is a Zen thing, like "How many babies fit in the tire?"