I'm not a big video game player, but I bet many of you are. Do you know of any video games wherein you advance in the game by leaving the game cartridge or disc on the shelf and doing something else? Do video game producers get angry letters from their customers about how they are being punished for not advancing in the game when they're not actually playing it?
In many MMOs, including World of Warcraft, there's the concept of 'rest' XP. If you leave your character in a safe place - such as a city or inn - when you log off, you build up a buffer of time during which, when you next play the game, your characters' actions will all earn XP at an increased rate. It's there for precisely the purpose of providing a way of helping players with busy schedules to not fall too far behind their friends who are able to dedicate more time to playing the game.
There are also social options for helping players catch up - players who are members of a guild may ask higher-level allies to help them through game areas or quests that they're under-leveled for, to quickly boost them up to match their team-mates.
The 'punishment' for being underleveled in an MMO is participation - particular dungeons or PvP areas have prescribed level requirements, and if you lag far enough behind to not meet the requirements, you quite simply cannot join your friends in that activity.
This isn't a criticism of leveling everyone up at the same time even if they don't play. That certainly works if you think there's value in having all the PCs be the same level. It's just perhaps a way to look critically at the assertion that not getting levels when you don't show up is some kind of punishment. Why might it be seen this way in D&D, but perhaps not in a D&D-like video game?
There aren't a lot of really D&D-like video games - not in the sense of being games where groups of players get together with a particular set of characters and play purely co-operatively across a wide range of power levels. The only examples I can think of off-hand are very old titles such as Baldur's Gate or Neverwinter Nights, or the more recent Divinity Original Sin II with its DM Mode. Given that the latter game uses DM-granted XP rewards for advancement in DM Mode, the choice of group or individual leveling is up to the DM.
The older titles (and we're talking up to 20 years old for some of those) pretty much relied upon full 6-character groups, and if a player didn't turn up, their character would be taken over by the hosting player as I recall, so it wasn't an issue. But you could technically bring in a much lower-level character to one of those - and I can tell you, it'd feel like a pretty darn hefty punishment to have to play that character. The NPC AI does not discriminate between characters, and a significantly under-leveled character would have a life expectancy measured in seconds in most of those combat encounters.
It seems to me there are good reasons to want to level everyone up even if they don't play and this supports particular play experiences. But to say doing otherwise is some kind of punishment seems like a very weak argument to me. I think you can make the case for your particular approach without it.
The thing is, whether or not it feels like a punishment is going to vary from individual to individual. And if someone hasn't been in that position before, they themselves may not know the answer in advance. It'll only be when they're actually sitting there with a character a couple of levels behind the rest of the group that they'll really find out whether they perceive it as a challenge, or a punishment, or simply irrelevant.