Nintendo lowballed FY 2017 and overshot their goal. If the same market analysts at Nintendo are making these predictions, I think they'll hit it. There's a lot we don't know about Nintendo's 2nd half of 2018 and first 3 months of 2019.
But I can see both ways, expecting 18 million more Switch units in 9 months is a bit strange. I don't know if they're giving too much credit to Lets Go or what. Analysts are expecting Switch to be the best selling device for the year so Nintendo isn't necessarily alone when they are making these predictions.
I doubted Nintendo the last time that they made the claim of 16M units shipped, and that didn't turn out well for me. I'm hopeful that they do but as of now, it's headache inducing for sure how they'll do it. Let's Go isn't even a real mainline Pokemon game...maybe that helps or hurts their bottom line.