Synchro Summoning is the procedure of, now watch the difference in wording as opposed to "tribute", sending one or more Tuner monster(s) and any (or, in certain cases, a specified amount of) amount of Non-Tuner monsters (the exception proves the rule - there is only one Synchro Monster which does not need Non-Tuners as Synchro Material, which is XX-Saber Gottoms) to the graveyard.
If I used to many relative clauses, here in shorter form.
Synchro Summoning is the procedure of sending one or more Tuner monster(s) and any amount of Non-Tuner monsters to the graveyard.
Conclusion: Synchro Summoning does not tribute the monsters - it sends them to the graveyard.
Tributing may be defined as the act of sending monsters to the graveyard in order to perform an action (a Tribute Summon or activation costs for effects like Exiled Force), but sending a monster to the graveyard is not considered tributing them; for example, Mystic Piper includes an effect which tributes itself, means you couldn't activate the effect if you took control of it with Mind Control, conversely, a card such as Rescue Rabbit (albeit it being banned) could have activated its effect - which is one reason why Mind Control is now limited.
There are multiple opposing scenarios similar to this, like drawing and adding a card from the deck to the hand, which I won't elaborate here.
Hope I was able to help.
