what they really mean is that prior to 1905 there were a series of Balkan wars during which Britain stepped in a stoped the unification of all the bulgar related linguistic groups from forming one nation. Macedonian, Bulgarians and northern greek Thracian Macedonian/Bulgarians. they wanted to be one nation and the british were instrumental in stopping that. this still causes ethnic disasters and strife in the Balkans. the greeks have tried to destroy the thracian Bulgarians from learning their ancestral language, serbs tries to stop madeconidans from getting independence, and there is till a slice of Serbia that speaks bulgar/Macedonian.
these are all related and mutually intelligible dialects. not separate languages.
all the Bulgarians I know are quite bitter about this, and blame the british empire.