I agree that it may be difficult for armin to reach the boat before them. But where you say armin is riding beside eren and mikasa before mom titan arrives, i believe you are refering to historia.
Right here we see Historia
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She's riding on Connie's horse (as she has lost hers for the moment), and Ymir follows behind them. Additionally, notice that they are noticeably far away from where the Titan is being launched too, and that they are moving a little off angle from Reiner's thrown target direction.
At the bottom left panel of this page we see Armin and Hannes.
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The Titan that was thrown crashes right next to them. There is further evidence as Jean is depicted right beside Hannes as well on the page just prior (we also see the Titan coming down overhead).
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Also, there could have been a chance it was Historia; however, I'd say the hair doesn't match, but more proof is Historia is basically laped over on the horse laying over to where her head sits on the horses right. The image of the person looking back has their head turned to the left of the horses side, and is sitting upright (not to mention Connie nor Ymir is anywhere nearby).
This also shows that Armin is moving in the direction that is opposite the one the Mom Titan is shown walking at the end of the chapter.
Mom Titan ==> Eren and Mikasa <=== Armin, Hannes, Jean, etc.
I tried to find it, but there's even a picture of the Mom Titan by other Titans... but after spending too much time with no results I've given up.