Huh?
What do you think you are telling me that I don't already know?
The first part of your post suggests that you don't understand how vote splitting works.
If that is the case, they could have achieved the result they wanted without running at all.
If the Brexit party didn't contest a specific constituency, then the Labour voters there who were tempted to vote for it are more likely to vote for their second choice (quite possibly Labour) or not vote at all. Your contention that they could split the Labour vote without contesting the election is to a large degree inaccurate.
I know what system the UK has. I asked, in the quote, what seats the Brexit Party affected?
Go the BBC 2019 election result website:
https://www.bbc.com/news/election/2019/results
Click on the "Change" button near the map and look for seats that were won by Con.
Click on the "Show results" on one of them.
Take a look at the amount of votes that Labour lost and compare them with the amount of votes the Con gained and the BRX gained. Some seats where BRX got more votes than the difference between LAB and CON: Durham North West, Blyth Valley, Burnley, Bury North, Bury South, Bolton North East, Heywood & Middleton, Dewsbury, High Peak,
Labour lost a lot of critical seats that they have held for a significant time because previous voters instead gave their votes to both the Brexit Party and the Conservative party, but also to a lesser degree the Liberal Democratic party and (to an even lesser degree) the greens. Had the Brexit Party not stood any candidates there, it's quite possible they would've been able to keep enough support to retain some of the seats they lost.
Edit: The amount of votes gained by the Conservative Party and The Brexit Party was both quite often around 50% of the total votes that Labour lost.