I think the concern is more that they have such a small moat. Their product seems too easy to copy. But given that they are first to market, did a very a good job with what they offer and have acquired a lot of customers, that is all worth a lot of money. Is "a lot" $1B? Hard to say.
The brand also becomes a reinforcing moat in an interesting way when you become a household name. When your employees think to themselves "I want to send a quick video update to team X" and they instantly default to downloading Loom, IT's decision for which vendor to buy a solution like this from is practically made for them.
