This is a very old topic for me, that I have discussed here and about over the years. Indeed a typical passive summer with make-up gain and a virtual earth summing amps will have similar noise floor performance.
Back in the 80s I developed a combining strategy using active current sources that avoided the noise gain (N+1) of virtual earth summers. My practical current sources (based on TL07x op amps) exhibited 10x the source impedance of resistance summers so reduced that noise gain roughly 20+ dB. Today using modern op amps and spending a little more money I could improve upon that, but so what? Digital combining is effectively perfect, and high quality analog summing can already be quieter than mic preamp or ambient room noise.
Back during the peak of the "Passive summing" fashion trend I suggested to a friend still making and selling consoles, to make a DC coupled mic pre amp input to sell to those passive summer fans.
JR