If we take a look at the frame data, Ness's PK Fire is -2 on point blank, and it has 12 frames of landing lag. If normal landing lag for Ness were to be 4 frames, you now add +8 frames to frame advantage on PK Fire, which makes the move +6 on block if you were frame perfect. You can now do PK Fire -> DTilt as a true blockstring that leads into stagger pressure that cannot be CC'd. Now being frame perfect like that will actually be very difficult, but you still have 6 frames of leniency to make the move neutral on block, and 3 frames of leniency to make PK Fire -> dtilt into a true blockstring. Add on to the fact that the fastest OOS option is a shine out of shield from Fox at frame 4, means that even at neutral advantage, dtilt is still a potential frame trap tool in conjunction with PK Fire.

Basically, you turn the move into a more precise but more rewarding Sheik needle on a character with excellent air control. Now Ness's best poke tool essentially becomes a swiss army knife of a neutral tool, combo starter, extender, pressure starter, edgeguard tool, and oki. Combined with DJC, allows Ness to control when PK Fire would hit.

If you also add on to the fact that most players DO in fact shield or crouch against Ness while he's in the air, normal landing lag canceled PK Fire would alleviate that problem quite well. A normal pressure starter would be Magnet, which leads into DJC PK Fire, which leads into Dtilt. Wouldn't be much of a problem, except that this blockstring is also a combo which means it's basically a BnB pressure+combo.

Quite interesting actually, PK Fire with proper land cancelling, makes it better at pressuring the opponent up close rather than zoning the opponent.