To be clear, most arguments I've read are NOT arguing for an increased multiplayer-limit along with an increased party size limit. The max number of players would still be 4; some players would control multiple characters in a 6-person party. I agree that playing BG3 with 5 other real people would likely be a...not enjoyable experience.
And again, Larian wouldn't need to adjust any encounters. Instead, they could scale exp so a party-of-6 is lower-leveled than a party of 4. All encounters could remain designed for a party-size of 4. Alternatively, they could adjust all encounters for a party of 6 as per @GM4Him's wishes, and then give smaller-parties boosted exp. Whichever.