The commentary gives a notably broad allowance for the emphasized Fajr Sunnah:
• if the person reasonably expects to complete the 2 Sunnah rak'ahs and still catch the congregation, the Sunnah may be performed first;
• the commentary even records that catching the final sitting can be enough to avoid completely losing the congregation for this purpose.
If the person reasonably fears missing the congregation altogether, the Sunnah is left and the Fard congregation is joined.
This detail should be presented as the source/commentary's rule, not converted into a general rule for every Sunnah prayer.