I read an article on this a while back that made me refrain from actually getting bees. I can’t find it right now, but the gist is that domesticated honeybees will compete with a lot of other pollinators (mainly solitary bees) over the exact same food sources.

However, the honeybees have a gigantic advantage in being supervised, housed and generally looked after by the apiary. Which will also employ methods to stimulate hive-growth, driving the hives demand for food.

That is something a solitary bee - or another pollinator depending on the same nutrition - cannot compete with, driving them away.

So, in a nutshell: adding bees to a place already rich in honeybees? Whatever. Adding honeybees into a local ecosystem not having them rn? That will drastically lower biodiversity

I'm no biologist, but as for why they're bad for other pollinators, yeah, what @frosch@sh.itjust.works said sums it up quite well.

I'd like to add that, to my understanding, they're actually relatively ineffective pollinators, too. They might do the highest quantity in total, but I'm guessing primarily because of how many honeybees there are.
I believe, the paper you linked also observes this, at least they mention in the abstract:

With respect to single-visit pollination effectiveness, A. mellifera did not differ from the average non-A. mellifera floral visitor, though it was generally less effective than the most effective non-A. mellifera visitor.

...but I don't understand the data. 🫠

As for why this is the case, for one, honeybees are extremely effective at collecting pollen, with their little leg pockets, which reduces the amount of pollen a flower has to offer.

But particularly when they're introduced into foreign ecosystems, pollinators that are specialized for local plants get displaced.
This may mean just a reduction of pollination effectiveness, or it could mean that the honeybees turn into "pollen thieves", i.e. they collect pollen without pollinating the plant.
Here's a paper, which unfortunately no one may read, but the abstract describes such a case quite well: https://pubmed.ncbi.nlm.nih.gov/20583711/