This is pure speculation and not revealed truth, so I wouldn't indulge that spin, but I think its very likely that you are correct - that someone with tabletop 5e experience was hired and brought in to write this campaign, or at least the initial draft version of it, with the nuts and bolts all pretty tight. Basically meaning that the major encounters were outlined assuming a standard 5e party size of 3-5 members, with monsters using standard stats, CR, the recommended XP progression etc. In other words, that the campaign was planned and created in PnP and then handed off.
I think it's also entirely possible that many of Larian's initial development staff had no substatial experience playing table top Dungeons and Dragons or even Baldurs Gate 1 or 2 when they landed this project. Perhaps they are people with Ragnarok's predisposition, and no particular attachment to D&D systems or to the earlier BG games in the series, beyond just a general interest in developing cool fantasy rpgs? Maybe several of them were simply too young to have had any direct experience of the original games, either as games or as a cultural phenomenon. So they just don't really care about that stuff as deeply and also don't see how much extra work they are actually creating for themselves by going off script there.
Lets just say for this thought experiment that they had a pretty dialed PnP campaign thrown into their laps and were then directed by their bosses to "make this work!" with the pre-existing game engine. I can easily imagine how something like that could happen. Also, because the existing game engine was not particularly well suited to handle D&D rules they started changing everything around immediately so it would at least begin to take shape. Mixing and matching, throwing out encounters, adding monsters or nerfing them -heavily tweaking and adapting everything to suit their existing systems, instead of first designing appropriate new systems that the original campaign draft assumed would be in place, before players were invited to play.
Maybe they released it a bit to soon into EA, probably as a cash grab, to sustain the project under heightened fears of the possible economic fallout of a world crushing pandemic in the offing. You know, to get at the money before everyone was broke and out of work and also while we were all still locked down at home and searching for things to do with our time. So this is why we ended up with a half baked potato.
If they pushed it out too quickly with underdeveloped systems - such that all the work of prebalancing encounters by the books got thrown out the window in the scramble to get something out the door that would be serviceable as a demo - now they are stuck in the position where half their playerbase thinks that what we're seeing is fully intentional. All part of the grand plan to make DOS2 Faerun, and that what we see now is somehow already balanced and tuned. While the other half thinks it's still a hot mess waiting to be untangled, once the real D&D systems are in place.
Right now players who have no real affinity for the D&D or BG part, but just want a shove-em around tb action game with a D&D story might think it's rad and 'don't mess with the winning formula.' Whereas people who wanted something more mechanically consistent with D&D this couldn't possibly be right, cause it doesn't behave like a proper campaign using the standard standards should play.
What they desperately need is a game mode called "Action" so they can do whatever the hell they want with it for their Divinity fans, and another game mode called "Classic" which tries to appeal to the people who came here expecting a more traditional D&D campaign and aims to do everything it can to put the first campaign draft into proper practice. Again all wildly speculative. Such a campaign might never have really existed beyond an outline, but I can see the logic and the circumstances that might have allowed such a situation to arise.
When the game comes out of EA and they've basically ignored the Classic audience to go with party of 4 and a paltry 8 companions. I'll know at that point that they've failed. Until then I have to keep arguing in favor of the disaffected cohort here who wants the game to work for >4 with the normal stat spreads and standard systems.