AdGuard DNS
a DNS AdBlocker just stop a request to a specific domain. If youtube serves video and ads from the same server you do not know which one is an ads and which one is a video on the DNS level. Furthermore you have some devices hardcoding DNS server (google homes are common for calling google DNS directly instead of getting the DNS from the Router) which makes it very hard to use DNS level AdBlockers.
Google spends a lot of money mixing ads and legit content to make it impossible to block, it is not safari "letting ads through" is more like safari cannot identify ads anymore.