[KirillPanov]

Not really. If the attacker has code execution on the target servers they can just blast out UDP packets (with erasure coding for bonus points). The DDOS will jam up the inbound link to the target server, but the outbound link will be able to send out unacknowledged UDP packets just fine. Can make them look like DNS requests if you're paranoid.

If the attacker doesn't have code execution on the target -- in other words they just found a way to make some web server cough up data that it shouldn't -- then this won't work.

[viraptor]

This sounds like an action movie sequence. See the sibling answer. DDoS gives people reason to look into logs/metrics. If you get caught by an automatic check, you fail either way. If not, you move from an "everything normal" scenario to "we're looking at ways to kill traffic" scenario, which is not beneficial for exfil.