The Avengers annual doesn't need to be in the MTIO annual but the MTIO annual has to be in the Avengers annual.

By the way, the reason the Thing and Spider-Man appear was because Archie Goodwin didn't know in advance where he could publish the end of the saga so by including Spider-Man and Thing, he could publish it as either MTU or MTIO. This was the period of Marvel history where cancelled titles ended their storylines in MTU or MTIO. Warlock had already appeared. Would that be sufficient to include the MTIO annual in the MTU Masterworks?
There were other Annuals which included either Spider-Man or the Thing as guest stars in case the main series was cancelled. I remember a Nova/Spider-Man team-up.