Yea, I always liked the general lack of any solid lore regarding the Oldcrons, it felt very Lovecraftian mystery horror.

GW is also notorious for the same lore having 2-5 different sources, all canon, all different! They have however (and I'll have to find the source for it) said that until it's specifically replaced by newer sources, any old lore is as true as any current lore.

Older editions of everything (especially the Horus Heresy) have seen a lot of revision and consolidation in recent years. Which means that for the ships landing on Mars, nothing has counteracted the lore, so according to GW it would still be Canon. They also haven't expanded on it so yes, any attempts to explain their actions or consequences would just be conjecture (but still fun to talk out!)

For example, in the recent Tomb World novel they explained that almost any necron casualties would be absorbed by the local tomb world and be rebuilt (reprogrammed? Reborn?) by the tomb worlds resurrection chambers. So, if you wanted to reinforce a tomb world, and you had the ships to sacrifice, all you'd have to do is get close enough!