If Biden wins the election in 2024, his record in office could affect the results of 2028, either by causing people to sour on the Democratic Party, or convincing more people to vote for the Democratic Party after a successful term. Therefore, the probability here should not necessarily be the same as Will a Democrat be elected president in 2028?57%
No matter who wins the 2024 election, the 2028 election is likely to have no incumbent, since either Joe Biden or Donald Trump would be prevented from running for reelection by term limits. Therefore, the difference between this market and the unconditional version can be compared to the difference between Conditional on Trump being elected president in 2024, will a Republican also be elected in 2028?40% and Will a Republican be elected president in 2028?42%, since neither one is influenced by an incumbency advantage or disadvantage - both are purely measures of whether a certain candidate's term will strengthen or weaken the chances of their party's next nominee.