In short, yes. If your "Donation Success" triggered messages are enabled, your offline donors will still receive a thank you message from WeGive. However, you can always use the audience filter rules to ensure that your offline donors do not receive a "Donation Success" message.

To disable "Donation Success" messages for offline donors, create your default message with the rule group "Payment Method Type is not offline."

Then, make sure to pause your triggers that will send for "No event filters added" and "No audience filters added."