The MB is saying that even in the view of those who say
‘Reicha milta hi’ – (aroma is a concern regarding year-round kashrus – in yoreh de’yah)
Even they agree that when you double-wrap, reicha is not an issue to begin with. Not year round (yoreh de’yah), nor on Pesach.
(This was prevalent way back when there was only a shared community oven)