Good question. This is one of those times where you have a difference in “theory” and in “practice.”
Yes, you’re definitely correct, sliding from the 3rd fret for a Dm chord would be better theory wise.
But what I found when I was playing the song is that you need to do the slide with your 1st finger, not your 2nd finger (your 2nd finger is already on the 3rd fret).
To me it didn’t feel as comfortable to try and get my 1st finger onto the 3rd fret while sliding.
I try to make my tabs as close as possible to the way I really play the song.
You could certainly try the 3rd fret though and see if you can make the slide sound smooth.
Same thing happens in the 2nd chorus, sliding from the 1st fret on the 3rd string (over F chord) instead of 2nd fret (better theory wise). But in practice, I found it was easier to move my first finger up and start the slide. Which means you start the slide from the 1st fret even if it doesn’t work as well with theory.
Hope this makes sense. If you have any other questions let me know.