Hi @christiananton - I think it's not showing as a mention counter only for the account that created the webhook. So as a temporary work around, you could try setting up the integration with the @pseudouser account, and then sending the message to the DM channel of the account you want to get the counter indicator.
I'm not sure if it's expected or not that the counter doesn't show for the person who created the webhook, but it's definitely a bug that mentioning @username doesn't cause the counter to show up in the DM channel.
Edit: Filed a ticket here for the bug https://mattermost.atlassian.net/browse/PLT-4841, with the expected behaviour that all webhook messages sent to a Direct Message trigger the counter (like a normal direct message would).