Si has quite a few meanings, one of them being the introduction the premise in an inference (see II a in the link above). Using que remove the deduction aspect. Using seulement parce que not only remove the deduction aspect, but also leaves the impression that the person should not be working for Google (the version with si giving the impression that merit was not enough, chance was also needed).

As it happen, in this particular sentence, que is also possible with, although it would call for the subjunctive in the subordinate:

C’est donc un coup de chance que tu te sois retrouvé à travailler pour Google.

And so, to answer your question as to “How using si in this manner differ in meaning from using que, I’m with Simon in that, choosing one or the other makes absolutely no change in the meaning of the sentence, whether facially or as far as emphasis is concerned.

I’m going to guess your confusion may come from the way if is far more likely to express a possibility in English. Indeed I don’t think this particular sentence would translate naturally to a construction with if. However, French si expresses more generally a condition rather than a possibility (Grevisse, Bon Usage 14th ed, §1153 and following), and a condition need not be hypothetical at all.