If this is true, the case then becomes "Meta was a monopoly from start_date-tiktok_date" which isn't a very meaningful claim since they are not arguing it is a monopoly to be broken up.
Anyways, I disagree - this is not the case. If you read the filings and their slides, the FTC argues Meta is a monopoly in the personal networking space.
They essentially carve a market out of thin air to selectively exclude Snapchat, TikTok, and Shorts. The judge has understandably called this for what it is.
It was a phenomenally poorly litigated case, most experts at the time doubted it would succeed, but it did wonders for Lina Khan's popularity. Seems to have served her well with NYC and all.
The original claim was centered around the timeline of purchasing Instagram and Whatsapp. TikTok came much, much later.