You are correct that using a hash does not protect an individual from other users discovering that they can contact them with Signal, which is to be expected because that's the purpose of this feature. If you suspect that Bob, with phone number +15555551234 has Signal installed, you can verify that by... typing Bob's phone number into your contacts list and installing Signal so you can send messages to Bob.
For the purposes of entropy, you need only consider 10 valid choices for each symbol of a phone number so it's closer to 33.21 bits (10 * (log(10) / log(2))) and smaller still when discarding impossible area, trunk & subscriber numbers.
So given than 80 bits is much bigger than 30-40 bits, if I know someone's hash I can very easily narrow down their phone number to one or sometimes two candidates.
You are correct that using a hash does not protect an individual from other users discovering that they can contact them with Signal, which is to be expected because that's the purpose of this feature. If you suspect that Bob, with phone number +15555551234 has Signal installed, you can verify that by... typing Bob's phone number into your contacts list and installing Signal so you can send messages to Bob.