It is purely a semantics game.
The North American tour of Mamma Mia has been non-Equity and produced by Worklight for three seasons. This final season will be its fourth. Technically speaking, each year has a separate tour; the show goes out on the road for a full season, it closes and the contracts all end, and then a few months later, the tour is reassembled with some venter and and some new people for a full rehearsal process and it goes back out on the road, then repeats the process. Therefore, it is not inaccurate of them to call this a new Farewell tour. The average audience member just would not know that unless they paid attention to the full schedule or know of company contracts.
That said, it is different an the previous second national tour, which downsized two twice, the second involving an entirely new set. And made little distinction in their press releases. But this is BWW, and people will express discontent whether a release too little OR too much.
Also of note, aside from a few minor details (moon effect, shape of the wings), this tour of Mamma Mia, designed for short engagements, is very accurate to the flag ship West end production's current design, so hey...