Seriously welding seatposts isn't the best idea in the world as you're likely to get some weld distortion – you want a perfectly cylindrical post. In frames the seat-tube is reamed after welding (and heat treatment) to make sure that the inside of the seat tube is perfectly round.
I appreciate that there is always a level of distortion with welding but it won't make any difference on a seatpost coz it would only be in the area of the weld and you'd never put it that far into the frame.
Also if it's welded correctly (ie with the appropriate gaps & bevelled edges) so you get the right depth of weld penetration then the weld will be as strong as the base metal.
If you're gonna be a smart ar5e then be a correct smart ar5e 😉
Loovely seatpost by the way