Calculating SFDR from FFT of ADC output
I would tend to believe that there is an indeed a flaw in the approach as the datasheet SFDR is achievable given the test conditions which are usually specified in the datasheet as well.

23 dB is SIGNIFICANT and I suspect your sine wave is not really a sine wave and you are seeing a harmonic (or an aliased harmonic). 

Here are some possible culprits, some of which you may have ruled out already:

You did not specify what frequency the next highest peak was, but harmonics are usually excluded from SFDR.  (400KHz, 600KHz, 800KHz, etc)

Did you sufficiently filter the analog signal prior to sampling? If not any higher aliases will fold into band. (Changing the sampling rate and retesting, not the FFT size, would give clues if this is occuring with such a strong spur).

Did you observe your test signal in the analog domain to see if it is cleaner than the SFDR you are trying to measure? If you did not look at this (with a spectrum analyzer for example) is possible your test signal itself has such a spur. 

Similarly, did you confirm that your clock signal is cleaner than the SFDR you are trying to measure? Spurs on the clock will translate to spurs in the output signal as well.  

Did you set the input signal level appropriately and is your ADC interface per the vendor's recommendations? Typically the best SFDR is when the input signal is one to two dB below full scale where clipping starts to occur, otherwise non-linearities the front-end of the ADC with start to reduce SFDR. If your input signal was too low then you will clearly not get the SFDR of the datasheet (typically the spurs stay at the same level, so you maximize SFDR with an input level as stated above).

As far as your resume, I don't think anyone will ridicule you given what you accomplished in this project, as long as you present what you did, what you think you understand from your efforts and admit openly that you still have a lot to learn and show that you are actively learning and interested (which you are by coming here!). That is probably one of the best skills someone new to the field coming out of an internship can have. I think it was a great project you got to do and would be impressed hearing more about it. At your level, if I was a hiring manager interviewing you, I would look for your interest and enthusiasm and complete honesty in what you understand and willingness / ability to learn. If you thought or acted like you knew something that you didn't really understand, that would typically come across as a big negative and a reason to move on to the next candidate for a technical position. Best to "under-promise and over-deliver" by immediately saying you are not an expert, you still have a lot to learn etc and then while you have the floor dive into what you did and what you learned and how excited you were about doing it. (My 2 cents being on both sides of that fence).