this discussion is stuff for quadruple headaches if you’re an actual game developer. stop saying “this is engine xy because the metal is more shiny”, or “color grading” or “effects”.
first off: havok is a widely adopted physics engine used in the sc2 engine - its not a game engine on its own, its a “standalone component” that just handles the physics.
now on to the “different engine” thing.
wc3 and world of warcraft share the same engine. half life 2 and apex legends share the same engine. fortnite and hellblade: senuas sacrifice share the same engine.
the differences you describe come down to literal single digit number changes in the code or shader setup. maybe you dont even need to change anything at engine level, just the art assets and BOOM ppl will call this a “different engine”.
its impossible to eyeball the engine, except for you are an expert for working with these exact engines, knowing its quirks and very special, hard to spot characteristics of handling certain things - and this gets even harder if the developers actually know what they’re doing to mask these effects. i know the 2004/2007 version of source engine alot (since i spent roughly a decade working with it), and it is impossible for me to look at apex legends and say “yes, certainly, this is source engine”. usual giveaways for the source engine is the way cubemaps are handled, the phong and fresnel lighting effects, the animated textures, the fact that certain shader effects cannot occur simultaniously, the special way source engine usually renders this “half dynamic” lighting and shadows. in left 4 dead 2 or even CS:GO this is dead (haha) obvious, but apex doesnt show anything of this in any apparent fashion except for the animated textures - this goes hand in hand with a different art style; i couldnt go ahead and pin point the source engine here by looking at a video - heck, i’d probably have a really hard time telling whether its source or not by actually playing it - and im a 10 year veteran working with that engine full pipeline, that means from scratch to animated, textured model ingame. since i already know its the source engine, some attributes strike through, like the snappy responsiveness, some visuals are somewhat typical for source - but still it could be anything without looking at the game files.
so the whole “this compressed youtube video of a streamed pre alpha gameplay clearly shows its a different engine” statement seems a lil bit ridiculous. im not saying its the same engine, im saying you wouldnt be able to tell by compressed video material in the first place.
from a developer standpoint the 2018 demo probably is a work in progress testbuild of the updated wc3 engine to test and showcase the new graphics pipeline of the engine. it would not make any sense at all to port over all the quirky gameplay mechanics from wc3 in a different engine to have a slightly different look to it. you’d spend months doing just that. for what? theres no use. the final game will be the wc3 engine anyway.
blizzard probably decided some parts of the 2018 demo werent the direction they wanted it to look, or certain effects werent viable in certain aspects aaaaand they’re gone. like the occlusion effects that constantly crashed the game in the 2019 beta. maybe its not worth fixing it and it gets thrown out aswell.