I'd put forward that lance is a misnomer in that last sentence based on everything we know from other sources about lance strikes. Since it was enough force to level a city block, we aren't talking about precision weapons. Next, we know that such precise targeting of lances on targets that small is not possible (see epic, this has not been changed and is still official. Also IAV2 the original damacles strike always scatters). Finally we know from epic, BFG, and even the old damacles rules that the lance strike causes very little collateral damage. For example, the pinpoint attack doesn't harm other units even if they are in base contact. Also the ordnance template used in 40k is only slightly larger than a rhino. And in BFG lances are described as punching a hole in the target or cutting it in two not blowing up around it like WB.
Since the plantstrike fluff you posted is the only bit calling it a lance yet the description doesn't match with any of the existing descriptions of lance strikes, that means you either accept it's a lance and the description is wrong or that it's not a lance and the name is incorrect in the fluff.
Sebastius, the marine commander who calls in the strike survives, despite being fairly close, possibly only a few yards away, as well as other space marines with him, as the strike comes in. It hits a greater daemon of Nurgle. That's a much smaller target then a titan, admittedly, or it's a REALLY BIG daemon (description is unclear, though it picks up a landraider and squashes some people with it, and swats assault marines 'like flies'). Three separate strike cruisers are firing lances at the target. And three lances firing simultaneously would probably take out a city block, if one fired at the building on the end, one fired in the middle, an one hit the other end, though in this case they're all hitting the same target.
Strength of a weapon has little to do with it's precision: a plasma scalpel can burn right through you, but they're used in delicate surgery.
This is my point, a 'lance' attack merely means that it's a precision attack for that game system. Not that it's the one weapon across all scales. A lance attack on the scale of 40k does not equal a lance attack on the scale of epic let alone a lance attack on the scale of BFG. So a 'lance' is not a particular weapon, it's a classification that differs according to scale.
Well, let's see... it's a space marine strike cruiser... or three... firing their lances... at a ground target... which is vaporized.
Eldar did not pop out of the woodwork to fire brightlances. The text is VERY (for once) clear that this is a lance strike, fired by strike cruisers, not allied IN, not mysterious unknown escorts, and not a BC, and not the property of traitor marines. AND it's ENTIRELY SANCTIONED THE INQUISITION. It is not Black Library. It IS an offical canon GW rulebook. A NEW one even.
And since when are SCs equipped with plasma weapons (since a single plasma battery is a str 5 [at least] weapon? [And, more when was the last time a plasma weapon was used as a planetary bombardment weapon? Since according to fluff they're disrupted by a planets EM field, IIRC])
BTW: there is no lance strike for specifically for Planetstrike.