MTA is too siloed. But Transport for London? It's very siloed even from Thameslink, let alone other National Rail (commuter) services, other than Crossrail. It took 7 years go roll out Oyster to the main London commuter lines - and 16 years to get it running on Heathrow Express. And even then, they never designed Oyster to handle all the existing fare London-area zones outside TFL - so you can only use Oyster Debit/Credit for some stations - including on the Elizabeth Line.
Also remember there's no free transfer from TFL buses to the Underground, Overground, DLR, or National Rail; not sure about Trams - I haven't used those.
That is true, I didn't fully interpreted the comment correctly.
BTW, not related to this, there seems to be a lot of confusion here between fare integration and free transfers and identical pricing. Paying an extra amount to transfer from GO to TTC, but not a full fare is fare integration. Having equal fares for travelling on both GO and TTC is a different animal.