As to what a difference between 6sec RTwP and IgoUgo Turn-based - those are different rulesets - the difference should be self explanatory.
Yeah, I agree. His claim was that RTwP is the better choice because it can also be TB by only using an auto pauses after every 'round', which may be true with a certain kind of TB. However, doing so will not produce the kind of turn based combat Larian used in DOS and has shown to be in BG3, as really should be clear to anyone posting here.
Actually, that is not exactly what my claim is. But that isn't important atm.
If BG3 automatically pauses each initiative round so the player then the computer gets to issue orders to eligible characters the result would be IDENTICAL to RTwP set to pause at the end of each six second round instead of at the end of each one minute turn. In BOTH cases each game would pause every six seconds, and in BOTH games the only characters eligible to do something would be the ones who are eligible because of their initiative. This of course assumes that the D&D rules BG3 will use still follow the same pattern of 10 six second initiative rounds every minute.
This assumption is probably the source of your disconnect with BG3 game concepts. The last version of D&D to use one minute rounds of 10 x 6 second segments was AD&D 2e, the last version produced by TSR, and the last with any sort of input from Dave Arneson and Gary Gygax, who invented the game and the RPG genre in general.
In 2e, as you correctly state, weapon speeds, spell duration and movement orders determined in which segment activities took place, and 2 actors could simultaneously act, possibly killing each other ( specifically noted in the manuals ). And the whole choreography was the responsibility of the DM, taking orders from all actors at the beginning of the 1 minute round, and determining how those orders would play . It was also explicitly noted in the manuals that most of the 1 minute combat round was maneuvering, feinting, non-productive attacks etc.
After WotC bought out TSR, and Hasbro bought out WotC, the new version, D&D 3e, completely changed the basic structure of game time. The combat round became 6 seconds, within which all actors were explicitly arranged by an initiative system that does not allow ties. Although choreography based on actor orders was still the responsibility of the DM, the more restricted combat round timespan led to 3e being much more governed by explicit rules about exactly what action(s) can be performed; and, of course, 6 seconds meant a much reduced potential for movement.
Ignoring D&D 4e ( which annoyed most of the player base ), D&D 5e has doubled-down on the 6 second combat round by removing the concept of choreography based on actor orders, which is no longer a responsibility of the DM. Instead, 5e has serialised actor-time within the combat round. Each actor ( in accordance with strict initiative order ) performs their allowed action(s), while the entire world around them looks on ( with the exception that there are a number rules-based cases allowing reaction-actions to occur ). Orders are not given until it is an actor's turn to act, so each actor sees the situation according to the results of previous actors actions ( assuming they have not been killed by such actions ). This is more akin to a traditional rules-heavy strategy game than Dave Arneson's original vision for RPGs.
If this above description makes sense to you, you will probably now see why BG3 ( which is using 5e ) will not look or play at all similarly to BG2. The original Infinity Engine games used a set of rules that were essentially for simultaneous action, that could be paused, either at breakpoints, or in realtime. BG3 uses a set of rules that essentially discards time and replaces it with sequenced actions. Some people like that, and some people do not. It would not be impossible to turn it into something that works in a more realtime manner ( which, arguably, Larian are already doing with reactions ), but doing so would lose some of the character of the 5e rules ( as, again, some feel is true of the BG3 reaction system ).