the essentials of tefilah - words or thoughts?

The aside in yesterday’s post about Eliezer’s tefila being an improper request because his words could have led to a bad outcome even though his thoughts were in the right place has an upside as well. The gemara in Ta’anis (8) darshens the pesukim in Tehilim (78:36-38) “Vayefatuhu b’fihem u’b’leshonam yechazvu lo… V’hu rachum yechaper avon…” to mean that even though the tefilah of the tzibur may be no more than mouthing empty words, Hashem still promises kaparah. This idea needs explanation – the essential component of tefilah would seem to be the kavanah and ratzon halev. What good is mouthing empty words without thought behind them? Why even theoretically should Eliezer’s request have led to wrong – doesn’t Hashem know what he meant in his heart even if he didn’t formulate the request properly? Something to think about.