My point is that it's merely the same 53% since september 2018. And the massive dump did not even affect it.
So far I noticed that some people think it's too early to explain why it didn't change, but that's what I would like to know
It's not too early to explain. The reason BTC's dominance didn't change much is because it tanked in quite a similar fashion as altcoins did. Usually it's BTC down 5%, altcoins down +10%, but the decreases were pretty even this time.
If everything goes down equally, the dominance won't move much at all. In other words, Bitcoin behaved like a shitcoin in the last few weeks. It didn't show anything of its usual strength, it kept tanking like there is no tomorrow.
The most interesting aspect to focus on is whether or not the similarities will be there on the way up as well. If not, it might very well be that altcoins will outperform Bitcoin and drag the dominance below 50% in a quick fashion.