I haven't heard of someone getting a deal through or because of Tapas, but that doesn't mean it can't happen. Wattpad authors have become fairly famous because of the attention their work generated on there. I'm personally planning on capitalizing on the attention my novel Object and Objective got after the Tapas staff shared it on New & Noteworthy (which basically made my whole year). It's absolutely a query-letter-worthy detail, even though I don't plan on taking that particular novel to traditional publishers.
I also found some relevant links about usage rights on the Tapas guidelines:
The Tapas self-publishing and traditional publishing statements: https://help.tapas.io/hc/en-us/articles/115005476687-How-do-I-publish-on-Tapas-
The Tapas Terms of Service (you'll want to scroll down to "Your Content and Conduct" and "Licensing"): https://help.tapas.io/hc/en-us/articles/115005545248-Terms-of-Service
The most direct answer to your question, I think, can be found right under "Licensing" in the above link:
"Tapas Media does not claim ownership rights of your Content. Your Content belongs to you and Tapas Media does not restrict you from using your Content for any purpose unless stated in additional terms for other programs or offerings Tapas Media may provide."
Basically, we're free to pitch elsewhere, if I'm reading this correctly. Whether or not the new publisher will request you remove your work from Tapas is up to their new contract with you.
Best of luck pitching professionally! I plan to do this as well, but I am a slow starter. 