They're thinking literal food based on the CONTEXT of the passage.
It's always interesting how the Roman Catholic likes to move to the symbolic vs literal in this passage. For if they didn't their entire theology around this passage collapses.
The passage is where Jesus is saying LITERALLY and He repeats is -->
John 6:35 I am the bread of life. Whoever comes to me will never go hungry, and whoever believes in me will never be thirsty.
41 At this the Jews there began to grumble about him because he said, “I am the bread that came down from heaven.” 42 They said, “Is this not Jesus, the son of Joseph, whose father and mother we know? How can he now say, ‘I came down from heaven’?”
Then Jesus REPEATS for a second time that this is LITERAL - John 6:48 I am the bread of life. 49 Your ancestors ate the manna in the wilderness, yet they died.
50 But here is the bread that comes down from heaven, which anyone may eat and not die.
51 I am the living bread that came down from heaven. Whoever eats this bread will live forever. This bread is my flesh, which I will give for the life of the world.
And now the crowd is openly rebellious - denying, like you deny, that this is for us to eat His body - 52 Then the Jews began to argue sharply among themselves, “How can this man give us his flesh to eat?”
And He REPEATS a third time : John 6:53 Very truly I tell you, unless you eat the flesh of the Son of Man and drink his blood, you have no life in you.
55 For my flesh is real food and my blood is real drink.
56 Whoever eats my flesh and drinks my blood remains in me, and I in them.
At no point are we saying this is symbolic because Jesus clearly says this is to be taken literally
and that's what all the early Christians believed - that the Eucharist is the Body and Blood of Jesus