A particular educational software program published by a U.S. government agency uses a variant of RC4 to obfuscate its data files (see Stack Overflow question). The variant of RC4 in question is identical to standard RC4 except at the beginning of the pseudorandom generation algorithm. The developer omitted the first two lines below (from Wikipedia's description) because he based his code on an erroneous implementation from Planet Source Code from around 2005.
i := 0 j := 0 while GeneratingOutput: i := (i + 1) mod 256 j := (j + S[i]) mod 256 swap values of S[i] and S[j] K := S[(S[i] + S[j]) mod 256] output K endwhile
Thus, i and j retain their final values from when the key scheduling algorithm ran (i = 256, not 0, and j = (j + S[i] + key[i mod keylength]) mod 256 from when i = 255 during the KSA).
In this case, the encryption provides no actual security because the key is hardcoded into the software, but suppose the developer had used the code to encrypt a secret file on his hard drive. Does this implementation error significantly weaken RC4's strength?