Sometimes guys just mean as "why do you all" kind of thing. I don't find it really offensive too and I think he was trying to referring to anyone who writes isekai.
Personally, I'm not a isekai writer. However, I can take a guess at why people want and like two write isekai. Maybe its easier to write an isekai since the person already has some characterization of how they were in the previous life so instead of just building the mc from scratch, they can build on already existing ideas of what the mc was like in previous life and get a more faster start to worldbuilding and implementing things from the mc's previous life so it sorta relates a bit to readers. Or, people who want wish fulfillment with the current skills and way they are and get to be able do something they normally wouldn't in a more realistic society so they want to combine what they already are and what they can get in a wish-fulfillment story. So like say an accountant kinda wishes to be more than just an accountant and go do heroic deeds or whatnot and build a harem or do whatever stuff that some may frown upon in society - well now that they are isekaied into another world, they are more able to do things they want freely and become an image of what they want to see themselves as such as being heroes of their world and their story and gets to be the boss when they didn't really got to in their regular life. This is just what I think of it though, so I could be wrong and there may be tons of other reasons.
It really depends how broad isekai can be stretched though. Could just transplanting someone into another world. So some people mentioned something like Narina can count as an isekai too.
Do we count an immortal reincarnated into the mortal world as different? Or no, because it just one plane above the other but still within the same universe? Wait nvm, I think I answered that question and its a no, since it is technically same world/universe. Just different level of planes.
I guess it just means portal to another world? Would that count? Like people whisking away to other worlds through portals a lot in their story, but not necessarily staying within that same world? Like marvel worlds or monster hunters nightmare academy? idk.
On an interesting twist to the general isekai and mc becomes heroic awed, thought of something...
We should have a granny or mom isekaied into a story about or the other side of those who have been reporting their loved ones missing when they are isekaied. the regular world feels like losing people espeically the grannies/moms and they want their loved ones back. One granny/mom goes isekai to find their son and pulls them out of it. Granny/mom shows up to isekai hero's door and exclaims here to take you back. But in the mdist of it all, she actually like grabbing a sword and sheild to fight and likes feeling invincible again (because in the isekai world, even though they are still in their same old body, they don't feel the effects of aging process due to isekai and yes, they got plot armor). Though, they eventually still drags their kid/grandkid back into the regular world, they misses and realizes why their loved ones wish to stay in the isekai so sometimes they now join in the fun (kid and grandprt/prt) just go together and go back and forth to other world for bits of fun, but this time together.