What about:
- No official invitations to foreign royals, ambassadors, or Commonwealth officials such as Governor Generals and Prime Ministers ?
- No Royal Family carriage procession with Horse Guards escort to or from the church ?
- Wedding at St George's rather than Westminster Abbey ?
I suppose British cabinet members (including the PM) and maybe the Leader of the Opposition will be invited, but even that is not a given at this point.
So, yes, it is a low-key event in the sense of being a semi-private ceremony, which nonetheless will be probably televised.
BTW, the number of guests is not that much relevant in itself. Carl Philip and Sofia's wedding probably didn't have 800 guests as Harry's wedding is rumored to have (I don't know for sure), but it was a much more high profile event as there were four queens and two crown princes in attendance , in addition to the Swedish RF properly and less senior princes or princesses representing the British and Japanese royal houses. Harry and Meghan's wedding will attract much more attention only because the bride and groom are much more widely known in the English-speaking world (most Americans probably don't even know that there are other RFs in the world besides the British RF).