If it's the website I'm thinking of, if you read it very carefully it doesn't actually say it can be used to upgrade doors with gaps of 11mm at the head of the leaf.. It only says they've successfully tested it "...on an old door with a head gap between 7mm and 11mm, the seal just expanded and remained in place for the whole test. Tested to BS476 Part 22 (1987), achieving an integrity of up to 66 minutes with an 11mm gap at the head of the door...". It's feasible that it did work (if you chose a smallish door leaf of suitably 'sympathetic' construction to test it on it's not beyond the bounds of possibility), but even if we assume it worked once on a particular door construction that is a long, long way from saying it would be capable of being used to upgrade a wide range of doors with this size of gap at the head.
In any case I've never had anyone convince me that you can reliably upgrade a timber door to give you 60 minutes F/R. Even the purpose-made fire resisting doorsets have a hard time achieving that rating - especially to the BS EN.