There may not be any deliberate backdoor in BitLocker however I think it's safe to assume that NSA has access to the source code, probably found some angle to exploit.
Regarding to state actors who have the resources to attack any system, I think it's important to make it as hard as possible, even if it's "known" they will find a way. Why?
Because it will drive the costs very high with years of R&D having as result that they'll only use new attack techniques on high-level targets and that means risk of revealing attacks goes up(assuming high-level targets are more sophisticated and spill the beans - as in Kaspersky case[1]).
[0] - https://en.wikipedia.org/wiki/Kerckhoffs%27_principle
[1] - https://securelist.com/blog/research/70504/the-mystery-of-du...
Most deployments use more tricks than that. I was just amazed at how long that one went on without compromises. Used same trick for desktops with custom client-server apps. These days, I'm working on CPU's that protect pointers & code while tools randomize (i.e. diversify) the application automatically. Orange Book era solution to secure networking, email, & databases still work with minor tweaks. I use hardened client-server schemes instead of web apps to avoid... more than I can count. TEMPEST safe + 100yds of space where applicable. High assurance security = build on what we know works & eliminate anything risky where possible. Obfuscations & diversity I add as you said to just slow down our shadowy friends with deep pockets & large staff. Works well so far.