Shepard was killed off to move the story forward two years without having to explain what the hell s/he was up to those two years. If ME2 was a reboot, it would tell the story of ME1 again but in a different way, not move the story forward.
A soft reboot does not discard continuity. That's not what it is about. It still recognizes the past. The past just matters a lot less than otherwise. The past isn't important other than to note its existence. Ghostbusters 2016 (lol) is a soft reboot, from what I've read about it. It still noted a continuity.
We don't need to know much about the ME1 Shepard to enjoy and continue onward in a ME2 Shepard.
On the other hand, the 'best place to start' remark about ME3 was somewhat junk, considering that arguably half the fun of ME3 is seeing the context and results of previous games; a few nice parts from ME1's experiences and choices, but largely ME2's. ME2 and ME3 were very connected, marketing for ME3 be damned.
It isn't out of nowhere when people claim ME2 is a soft reboot.
Personally, I kinda disagree with it. I'd say elements, big elements of it had a soft reboot, but Bioware was still dedicated enough to making a trilogy with enough connections between games. It is MEA that I predict will be a soft reboot*.
I think with ME2 Bioware kept in mind a few things:
-a few years between games, not just 1-2
-lessons learned about the IP and a desired refocusing
-a desire to grow the audience substantially, not keep things at ME1 levels of awareness and purchasing
So they 'killed and rebirthed' Shepard and his experiences of the Mass Effect universe. But they didn't kill and rebirth the Mass Effect universe itself. (In both cases more metaphorically, in the context of the franchise.) It is up to us to decide whether ME2 Shepard is primarily a new character and screw ME1, or primarily a zombie and ME ended with ME1 so don't bother anymore, or whether (as the canon of the series itself goes more with) ME1 Shepard died but came back in an at least slightly different form, but still he returned.
Elements of soft reboot, but within a trilogy that Bioware wanted to continue so it wasn't a full soft reboot. IMO.
*At the very least to start. I have to say that some of my crazier theories is that MEA starts off seeming like Bioware is totally running away from (but still acknowledging) the trilogy, but they instead have a twist part way into the game (half way, two thirds, near the end?) that ties MEA back into the trilogy in an awesome way. But so far we just have the strong impression that we're getting a soft reboot that doesn't want to deal with the trilogy at all, beyond not erasing its events. So far.
At the very least, I have to acknowledge that for such a long development time (4-5 years since ME3 or its last DLC!), its probably a good place to start off at - Bioware assuming the player is new to the franchise, one way or another.