Besides, it's just practice with それに〜

Grammar Explanation: In addition to with それに〜

Urushi plays three games of shogi simultaneously against the other members of the shogi club. After winning against all three members, Urushi fears she’s destroyed their confidence, and just before the upcoming tournament.

うるし:: 「いやいや！！みんなよくやってたぞ」; “No, no! You guys did great!”; 「それにただの練習(れんしゅう)だ」; “Besides, it's just practice.”

Key Points

それに adds another reassuring reason
- Urushi has already said everyone did well, then uses それに to add a second reason they should not feel bad.
ただの練習(れんしゅう)だ softens the loss
- ただの means “nothing more than” or “just”, downplaying the match as practice.
- The plain だ makes the line direct and casual, fitting Urushi speaking to clubmates.