Why does LastPass hash their passwords with PBKDF2 this way?
Can you explain what using PBKDF2 with 1 iteration is doing in the above?
  
  Is this the same as just doing something like a HMAC SHA256 hash?


This is mostly HMAC-SHA256, but you can declare how many bits of derived key you want. I assume it is used this way for ease of implementation (it's easier to have one working implementation of PBKDF2 than many primitives arranged in complex way).


  Are there any concerns with something like a compromised server receiving this hash and being able to derive anything from it about the original encryption key or master password (prior to the stored hash being computed)?


No, this is nearly impossible to do because as long as SHA256 is one way. It is very unlikely that SHA256 will be broken in such way (MD5 and SHA-1 have collision attacks only). If anything, this will leak if password was changed, because there is no random part added before it is sent to server, which is design flaw (unless they do something extra which you omitted).

Also, as SEJPM noticed, it is weird that only 5k iterations are done on user device and 100k iterations on their server. This would put server in better position to perform a brute-force attack. However, 5k iterations on client seem very low for most computers, and probably should be adjusted anyway.