The difference is that this is not an isolated case on telegram(you said it yourself: "some amount", which implies "limited"). At the same time, you can literally open up the app and with 0 effort find everything they are accusing them of - drugs, terrorist organizations, public decapitations, you name it. They also provide the ability to search for people and groups around you, and I am literally seeing a channel where people are buying and selling groups "800 meters away" from me and another one for prostitution, which is also illegal in my country. Meanwhile, see their TOS[1]. They have not complied with any of the reports or requests from users (and governments by the looks of it) to crack down on them. While 1:1 chats are theoretically private and encrypted(full disclosure, I do not trust Telegram or any of the people behind it), telegram's security for public channels and groups is absolutely appalling and they are well aware of it - they just chose to look the other way and hope they'd get away with it. You could have given them the benefit of the doubt if those are isolated("some") instances, sure. But just as in the case of Kim Dot-I-support-genocide-com, those are not isolated cases and saying that they had no idea is an obvious lie.
2000/31/EC[2], states that providers are generally not liable for the content they host IF they do not have actual knowledge of illegal activity or content AND upon obtaining such knowledge, they take action and remove and disable access to that content(telegram has been ignoring those). Service providers have no general obligation to monitor but they need to provide notice and take down mechanisms. Assuming that their statement are correct, and they had no idea, they should be in the clear. Telegram provides a notice and take down mechanism. But saying that there are channels with +500k subscribers filled with people celebrating a 4 year old girl with a blown off leg in Ukraine and no one has reported it in 2 and a half years after it was created is indeed naive.
If I have to dig through third party clients in order to trust a system, then it's clearly a shit system. Signal > anything else, especially telegram, which can burn in hell for all I care.
I don't see the difference with Signal here. In both cases, the only reason why you know that they do E2EE properly is because you (or somebody else that you trust) has audited the client code and confirmed that it does indeed do E2EE.
Nor does it require a third party client. In fact, in this regard, Telegram official client is slightly better because they have reproducible builds for iOS, while Signal, last I checked, does not (they do have them for Android).
2000/31/EC[2], states that providers are generally not liable for the content they host IF they do not have actual knowledge of illegal activity or content AND upon obtaining such knowledge, they take action and remove and disable access to that content(telegram has been ignoring those). Service providers have no general obligation to monitor but they need to provide notice and take down mechanisms. Assuming that their statement are correct, and they had no idea, they should be in the clear. Telegram provides a notice and take down mechanism. But saying that there are channels with +500k subscribers filled with people celebrating a 4 year old girl with a blown off leg in Ukraine and no one has reported it in 2 and a half years after it was created is indeed naive.
[1] https://telegram.org/tos/eu
[2] https://eur-lex.europa.eu/eli/dir/2000/31/oj