When making any decision, you have to:
Look at where the info. is coming from...
Many of the sources who say that Mondays can be very busy, are those who are speaking from personal experience. I have heard this time and again, from WDW visitors, CMs, etc....
Many of the sources who do not warn about Mondays are those who are going by 'algorithms'. We all know that statistics and assumptions can be used and twisted to back up any argument. There are too many variables here. For example, when the WDW guest are mostly family vacationers (during the peak vacation seasons...) then the assumption that Mondays would be the natural first choice following their weekend arrival might hold true. During times when the WDW guests are made up of more local traffic, and adults without school-aged kids, etc... Then this may NOT hold true... Also, the assumption that the EMH schedule would mean that MK would be less busy on Monday may well be just that, an assumption. Remember, many many people seek to avoid the EMH days... (off-site guests... guest who do not want to wake their family at the crack of dawn... etc....) Also, remember, that Disney has designed and chosen this EMH schedule to try to alleviate the natural crowds... In other words, if MK is indeed already usually busy on Mondays, then yes, they would choose to make the EMH on Wednesday, and attempt to try to draw EMH guests to another Park. They would NOT make the EMH at MK on Monday, which could possibly make crowding worse.
Personally, I go by the personal observations... followed by the logic above.
Also, about TGM, I have read all the promotional material about his site. He is supposed to be a Disney Veteran of many many years... former Disney VIP tour guide.... etc. Which would indicate a preponderance of personal experience. I would assume that this is why folks pay for his service. I would consider any comments mentioned to be both 'hear-say' and/or 'proprietary information'. Yep, both 'hear-say' and/or 'proprietary information'.
If somebody is inclined to believe TGM, and would like to learn more, then they should go sign-up for his site!
Remember, as many posters here often say... The advice here might be worth just what you paid for it!