As far as I read (I'm no expert!) they could check the SNI of the TLS handshake if they want.
But using the DNS of the ISP is handing them the data right in a way they can analyze/use them very easily afaik?
They route your traffic, hence they can see all IP addresses you communicate with. With a reverse lookup you can then usually find out the address too.
In the end it comes down to what your goals is. DOH indeed hides DNS queries from sniffers and your ISP, but the traffic between you and your destination is still visible for the ISP (unless you use a VPN or TOR).
If you only care about the content blocking aspect a third party resolver may make sense as @CrazyClown@lemmy.ca explained below.