You're confusing "landslide" with high odds that a candidate will win. They're not the same. One candidate, for example, could be given an 8:1 chance of winning by 4%; those are high odds but not a landslide. The polls that once heavily favored Clinton showed a narrowing gap as the election approached. People who want to denigrate polls weren't watching the trends; they would rather say that the polls were wrong because two months before the election they predicted that Clinton had a very high chance of victory. Three weeks before the election, Trump's own internal polls gave Clinton a 92% chance of winning. In his post-election analysis, Nate Silver said, "if Clinton had done only 2 percentage points better across the board, she would have received 307 electoral votes and the polls would have “called” 49 of 50 states correctly," and 2% is within the margin of error.