I believe you are correct that Luke's individual contribution to 'balance' in the force would be the same whether he was/was not located with the Resistance. However, if one accepts what Lor San Tekka says is accurate (which is a whole other question which has some great comments here) I would agree with what has been discussed elsewhere and say Lor San Tekka is looking to the future. A future in which if there is no Jedi Master to train the next generation the Jedi order would die out thus leaving an imbalance in the Force.
Put another way. Luke was in the process of restoring the Jedi order by training new guardians of the light side of the Force. Presumably in their role as peacekeepers and protectors of the Force to counter the actions of agents of the dark side of the Force such as Supreme Leader Snoke and Kylo Ren. I.e. to bring balance to the force by eliminating harmful elements. This interpretation of what it means to bring balance to the force would be supported by the comments of Obi Wan in Revenge of the Sith, “With all due respect, Master, is he not the Chosen One? Is he not to destroy the Sith and bring Balance to the Force?”
Accepting this premise, as Luke is believed to be the last living Jedi Master if he were to die before completing the rebuilding of the Jedi Order it might be decades, centuries or even millennia before someone strong enough within the light side could rise to the level of a new Master Jedi within the Force on their own to restore the Jedi Order or equivalent.
If so, then while it is conceivable that Luke could train new Jedi away from the Resistance, it is harder to imagine he could do it completely isolated the way we saw him the Force Awakens. Allying himself with what would be a natural partner in the Resistance, Luke would have help both in locating and protecting new padawans while he was rebuilding the Order.
Therefore, to Lor San Tekka's comment, he views not only Luke's survival, but his re-establishment of the Jedi order as necessary to restore balance to the Force. To do that he needs assistance from others such as the Resistance.